I recently bought an N64 with an expansion pack that had a BUNCH of problems. It runs wonderful now though, i just wished i had a single place to find a lot of the information i needed.
Video freezing with sound still going - for me swapping the thermal pads in the n64 did it
Graphic glitches, crash afterwards/ strong sound distortion, crash afterwards/ flickering, crash afterwards - for a lot of people cleaning the contacts on the expansion pack helped, for me swapping the thermal pad on the expansion pack completely fixed it
Random crashes with the led still on (no video or sound just instant black) - putting something small under the powerbrick so its slightly pushed up and cant move at all
Honorable mention: if the color red is really smeared/ ghosting you are probably playing a game on the wrong region console (for example NTSC game on PAL console)
Edit: i forgot to mention: playing conkers bad fur day on a flashvard with the expansion pack also caused freezing with aufio still playing, switching to a regular jumper pack fixed it