I was also getting it, spent all morning trying to fix it, then finally around 10am got it taken care of. I'll post for your a post I found via google and the info within fixed my stutter issue. Although if your computer is slow and that is why you stutter, this won't help.
To quickly recap my rig:
2 x WesternDigital 500GB in RAID 0 setup - normal HDD
(read/write sequential at 250MB/s; 4kB random read - 0.8MB/s)
Athlon x64 II 7750 - 2 cores clocked at 2.7GHZ
5GB of DDR2 RAM (800MHz CL5-5-5)
Windows Server 2008 R2 / Windows 7 - both OSs installed.
Gainward Geforce GTS250 with 512MB onboard.
DirectX diag: http://pastebin.pl/59427
tl;dr version of solution (not guaranteed to work, scroll down for more details):
1. Delete your agent.exe from ProgramData\Battle.net\Agent\ folder and run the game.
2. Exit the game, copy it somewhere.
3. Uninstall D3 from Windows -> run from the place you copied it.
1. The problem and what I tried and failed.
The problem is well known and is mostly described as the lag/stutter when loading new assets of the game. That means going into town, crushing barrel, smashing an urn, approaching and attacking new monster, using new skill, approaching new boss etc.
I have tried all this and it DID NOT HELP at all.
I haven't played before Patch 15 so I can't conclude on that.
I tried reinstalling my OS to: Windows XP, Windows 2008 Server R2, Windows Vista and reinstalled Windows 7 - no dice.
Disabled sound in-game.
Lowered sound channels.
Disabled sound device in BIOS (I have Realtek integrated).
Overcloced my CPU to 3.2GHz and went back to stock because it didn't help.
Installed newest drivers for all my stuff (that includes AMD Raid/AHCI drivers, soundcard, southbridge, video drivers)
Installed older drivers for all my stuff - especially AMD Raid/AHCI drivers and Video drivers.
Borrowed an SSD drive - it helped with the loading but the stutter was still there, just smaller.
Uninstalled D3-enUS and installed the enGB version.
Tried running with really nice VPN which decreased my latency from 225ms down to 105ms.
Disabled/enabled Prefetch/Superfetch in Windows 7.
I capped the FPS to crazy values like 5, 10, 15, 30, 45, 75, 100, 125, 150 etc.
Set all details to low, resolution to the lowest available.
Tried all combos with triple buffering on/off, vsync on/off, windowed and fullscreen.
Forced disabling Ambient Occlusion in Nvidia Control Panel (with use of nvidiaInspector). Default for D3 profile is forced ON.
Tried all kind of FPS limiters outside the game with v-sync turned off and on. Tried limiting it to different values like 58fps, 59fps, 60fps, 75fps etc.
Made sure that there is no 2nd Diablo 3 copy running in background.
Increased priority of Diablo 3 to all levels.
Increased I/O priority of Diablo 3.
Killed Agent.exe while playing.
Blocked all versions of Agent.exe installed in the Battle.NET folder in Windows Firewall and ran the game with Diablo 3 executable with -launch parameter
Defragged the hard-drive with Diskeeper.
Uninstalled Diskeeper as it intereferes with I/O with it's technologies like Intelliwrite etc
Defragged with Windows Defragmenter.
Tried all of this using Windows XP, Windows Vista, Windows 7 and Windows Server 2008 R2.
It all didn't help. It just made the game run different. Sometimes with tearing, sometimes slower, sometimes totally wrong etc.
2. My solution.
I borrowed some RAM and tried running it from RAMDISK. It was smooth as butter. But that's not an option. It's obviously blazingly fast. So I investigated further more and with only my HDDs.
That's what I did in order and it's repeatable. If I just installed the game from the downloader it would lag. If i did this - the lag and stutter is gone. If I removed it again and installed again - it would stutter again. Please don't tell me it's Superfetch picking up the game folders to memory because I moved them. I tried this with Superfetch completely disabled and on Windows Server 2008 R2 where there is no Superfetch. It's fully repeatable in my case. I can reboot as much as I want etc. It's not only one-time solution and you don't have to move your stuff everytime you reboot.
1. Installed the enGB version of the game.
2. Made sure it installed and patched properly.
3. Closed everything Diablo-related.
4. Went to 'C:\ProgramData\Battle.net\Agent\' and deleted EVERYTHING but 'agent.db' file.
5. Ran the launcher and after it was ready to play I closed it.
6. Deleted the agent.db from Diablo III installation directory.
7. Copied the Diablo III directory to somewhere else (anywhere).
8. Uninstalled Diablo from Windows.
9. Made sure that there are no leftovers in the Diablo installation directory.
10. Ran the game from new directory I copied the game to.
I did that to my friends PC which specification are totally different and it helped too.
That is the copy of what I read. Not my original info, but, it helped me and fixed all my problems, so the least I could do is to share it with you. Now, I still do get the occasional hiccup when loading a new area, but, not like before where everytime I went anywhere I hadn't been it would stutter to death. I didn't have the issue during open beta weekend.
Also, I didn't copy the files, I simply renamed the directory the game was installed to and then uninstalled from windows, but, of course files were still there in renamed directory. Then launched the game & nirvana!