I was driving to work today, and I was at 4,000 RPM. All of the sudden the needle dropped to 0! The 7 was still crusin fine, then the tach needle shot back up.....What is wrong? Is the tach plug coming out?

 

Assuming you have something other than a 79 with the factory point ignition, check the blue wire connector on the trailing coil. This is where the tach gets it's signal. Also check the trailing ignitor connections, they may be losing connection. Could be a failing ignitor, too.
If, on the other hand, you do have a 79 with the factory point ignition, check the blue connector on the leading coil.
See how even a minor detail such as the year of the car, can have major impact on the troubleshooting needed???????????????????