bad ground on inside of amp?

Anything is possible...

you seemed to isolate the problem by moving the ground wire...Is it connected well at the terminal? I have an amp that has no terminals for the power, they come through a gromet and are soldered directly to the board inside. Is this the type of amp you have?

Don't assume the ground is good at the body. The car has been vibrating and shaking for four months, plenty of time to free a poor connection.

If the ground at the body is bad:  Undo the ring terminal, find a good spot on the sheet metal to drive a couple of self tappers through without hitting the tank or lines, interior trunk metal is better if the screws don't go through to the outside (prevents weather corrosion) I also like to ground up top since I'm always encountering cars with water problems in the trunk area. shave some of the paint off with a small whiz-wheel, dremel...ask your girlfriend to do it with some 50 grit....what ever you decide to do, just get it to bare metal!! 

Now if that's not overkill enough, cut that ring terminal off...chances are that it sucks anyways. Get yourself a bare metal ring terminal of the correct gauge and ring diameter (it's ok to secure it with 2-3 tappers, it doesn't need to look pretty) Get gold, copper, whatever...it makes no difference to me. Crimp the terminal, get your open flame butane or propane torch, sweat the lug on there with some regular rosin core solder.  You can flux and tin it if that's your preference.....rosin core 60/40 or 63/37 is just as easy.  If you're real good with the open flame soldering, the insulation damage should be minimal....either way, that's what electrical tape and small zip ties are for, because that's what you are going to do anyways! You can shrink tube or fit a cable boot too, but tape and ties are always readily available.  

Both metal surfaces should be scuffed a little, add some dielectric grease and attach your ground with at least two self tappers. A star washer or two won't hurt a thing...

Now, this may sound like a redkneck install from hell, but I can assure you that when it is done under a panel or carpeting it makes no difference how it looks. Electrically, it will outlast the car.

If the bad ground is in the amp, again a small amount of 60/40 rosin core....but skip the propane, all you need is a 15 watt Weller pencil type iron here ;-)  

Best of luck,

FlynnTech

Disconnect the speakers and see if the amp will power up and stay stable by itself.  If so, then there is nothing wrong with your power, ground or RCAs.  If not, then yes, it sounds like the amp is defective.  What is it and what is it running (make/model/wiring of amp and speakers)?

What has changed in the "last 4 months or so" in your system (or in your car for that matter?)  Have you added or taken out anything?  Have you adjusted anything?  Has the car had maintanance done on it?

 if the amp is the problem, it could be that any bouncing or shock absorbed by the amp could of loosened a solder joint on the pcb or it could of strees cracked a little and when it is shaken a bit it disconnects the tracer then returns to its rest position and completes the circuit.