Hi Jake!
So the TV do have signal coming from the N64, but the screen stays black?
Anyway, the issues you are describing can be caused by:
* Bad Jumper Pack
* Faulty CPU
* Faulty RCP
* Faulty RDRAM
Could also be caused by the capacitors. Check those first and see if they are still fine or not. You could also buy a recap kit and recap them just to be sure.
I would try this first to see if that solves it or not!