QTP sucks up a lot of memory, and a lot of times when you run a lot of tests like that, it doesn't let go of some of it, so you'll see in task manager, on the processes tab, QTP's memory usage increases as time goes on. This can cause all kinds of problems. Rebooting will help temporarily, but there is another thing you can do to help mitigate this issue.
On your QTP machine you'll see in the taskbar near the clock an icon for the Remote Agent (If its not there, then Navigate to Start>Programs>HP QuickTest Professional>Tools>Remote Agent). Right click it and click Settings.
You'll see a section called Cleanup and under that a setting for 'Restart testing tool after # runs'. The default is 0. Change this to 5 or 10 or whatever you find works for your test set. What it does, is it will run X number of tests and then restart QTP before proceeding to the next test. This will help clean up the memory and reduce or eliminate most of these problems.