Are you stopping and starting I between each request? Perhaps too many instances are getting started and the machine is running out of memory. Try leaving it open and navigating back to the start page for each request. To really assist you, we'd likely need to examine a RuntimeLog and have access to the machine. You can open an SR with support to get someone to look at this with you.
If you do open an SR, please post the SR number here for tracking purposes.
The following article will be help you in creating an SR-