Most likely it's the synchos on it's way out. If it's not doing it all the time, you might be able to save it. Try draining the transmission fluid and replacing it with Redline MT-90. We carry them in stock, they are not cheap, but they work.

If the fluid does not take care of the problem, you are better off purchasing a new transmission and having it installed. Expect the transmission to run anywhere between $100~$300ish, depending where you purchase it from. Again, we have tranmissions in stock and will also provide a warrenty.