Electrical Problem

Hello.

I have 72 Cuda 340 and have been having electrical problems with my dash guages. I'm not sure exactly when it started since unfortunately I don't get a lot of drive time in. When the car revs past 3k whether idol or moving, I lose my radio, alternator gauge starts freaking out along with my oil pressure gauge. The only gauge that doesn't freak out is my gas gauge. Everything is fine below 3k. Bad ground? My tach is wired together with my radio using the same fuse. I was thinking of disconnecting the tach? I'm not real good with electrical so I apologize if I'm not giving enough info. Any help is greatly appreciated.

Thanks!

I am guessing not a factory tach?

while adding a ground might not be a bad place to start, why not try taking the tach out first?

see if it helps.

also get a quick reading on your voltages when you rev, as the alt gauge is isolated from the rest of the gauges and isn't grounded.