Mysterius problem, file upload not working – “500 Internal server error”

I′m in big need of this plugin. Would be so greatful if someone could help me. Everything works great with the plugin for about 5 hours, after that it′s impossible to upload files. Only the filename shows up (no thumbnails) and when you click the file name “500 Internal server error” shows up. I′ve tried to re-install both gravity form and ajax upload plugin and that solves the problem temporary but after 5 more hours its back to “500 Internal server error”. Does someone know how to solve this?