I'll tell you the issue.... 78C. Core 2 Duo, and Core 2 Quads, like to stay below 72C, 70C preferably.
It could be a couple issues causing the temperatures to go that high. Either:
A.) Your cooling solution isn't as good as the previous owner's. This could mean Air flow, HS/F, or mounting.
B.) Too much thermal compound.
Overclock the right way... Start at stock. Plug in all the voltages and timings for RAM. Bump FSB 5-10MHz at a time, stress test for 20-30 minutes to see if it's semi-stable. If it is, jump another 5-10, repeat. Everytime it gets an issue, bump your voltage a click or two, and repeat. Just jumping up to a certain speed without any trial and error can be ugly sometimes.