Hi Ashley,
Unlikely to be a thread issue since 100/minute is less than 2 per second and it does run for the first 1.5 hours. Can you elaborate on what happens on each request and what backend database is being used if any?
You could open the task manager and watch to see if the memory being used by the web app continues to climb to an excessive amount during the testing, it isn't a definitive indicator of a memory leak but it may provide a valuable clue.
Regards
Parker