This seems to have fixed everything for me!
Add this to your htaccess if your host has php4:
AddType x-mapp-php5 .php
AddHandler x-mapp-php5 .php
I ran across the solution while messing with a plugin that requires php5 and I noticed that my website is hosted with php4.
Just set this up a few minutes ago, but everything seems to be running smoothly now. I can enable as many plugins as I want, too. Will report back if I run into any problems down the road.
So nice to have it working properly again. Can't believe I struggled through that for 3 months! FYI, PHP 4.4.9 is what my host currently uses. Should be fine according to wp requirements, but certain plugins probably prefer php5. http://codex.wordpress.org/Hosting_WordPress