Program really slows down

I have a worksheet that uses a lot of macros and formulas. Over the course of 30 minutes, the program slows down to a crawl. So much so that I have to exit Excel and go back in. Is there any way to clear the cache while I'm still in the program?

for one, I have a randbetween(1,20) that buffers for about 5 seconds before it gives a number.

Instead of having people just trying to guess, why don't you provide an example of what you are dealing with?