The problem with Zerg is always that Infestors are the solution. A nice mmm ball can only be defeated with infestors + X.
The bottom line is you HAVE to use them. You can play with all the other units but you loose without infestors at gold on up.
I hate them. Thats why im in gold.
You don't need infestors. Just use banelings+speedlings. If the marines don't split they die instantly, and if they do then they are more vulnerable to your lings.