I will start this by saying that I don't know what the "heap zone" is, but I'm going to make a couple of suggestions anyway. I've been watching the thread, and it was clear to me that more RAM wasn't going to solve the issue, but no harm done. If you are, though, getting close to using 3.5 GB's of RAM (allocated to Logic) then it will get unstable, but more won't help it.
Are you able to know how much RAM you are using at any one time? Activity Monitor will tell you how much you are using, as well as what is using it. I also use a litte app called Menu Meters which hangs on the upper bar and always tells me how much I'm using and what's free at any given time.
Also, is the RAM checking out as OK? You can find this out by simply opening "About this Mac", and going to "Memory", and you should see it say "OK". There are obviously more advanced ways to do this, but that's a start.
Also, and this is just a suggestion, you may want to look into another program I use called Leopard Cache Cleaner. It will provide still more RAM testing, as well as clear your caches and do a little thing it calls "Optimize Free RAM". I know you don't usually need to clear caches with Mac OS, but this time it seems like you may want to do so. Optimizing the RAM is another possible thing that could help and won't hurt, anyway. LCC also has a lot of other diagnostics that may or may not help you. You can get it here, among other places:
http://www.versiontracker.com/dyn/moreinfo/macosx/16494
Also, did you do the extended hardware test that is possible by booting up from your OS install disk? This seems like the time to me.
Just some thought, I hope any of it helps. Unfortunately I have no idea what the problem is. Good luck, L