Run the game in debug mode:
1) Watch the object count and see if it starts to climb - if so you know that you either have a loop somewhere that's churning out objects, or you're not destroying objects.
2) Check the profile tab and look at the percentages for the different systems - is there anything that's using a crazy amount of CPU time? If so that's most likely the culprit.
3) If neither of these tests identify the problem go back to your code and start disabling events one by one of the latest stuff you added in and run it - don't worry if it breaks other things - just see if it fixes the slowdown; this should help you zone in on the offending events.
Thanks guys! I've been testing the level over and over again, but I couldn't recreate what was happening on a consistent basis, and after the last 20 times I've tested it, nothing happened. If it ever happens again, these tips will surely help!