Bad ground on gauges, bad gauges, bad ground on body to block, bad ground on engine block, bad sending units... could be a host of things. Adding grounds is easy. Get some heavy gauge braided ones from NAPA. Bolt one from the frame to the engine block, from the body to the frame, and you can have the ground from the battery to the frame or to the motor, and everything is grounded together. Many times the factory tiny braided grounds have long broken off the body. Add some heavy ones back on, and it will only help. Check the voltage at the bettery when the cars running or have a auto store like NAPA or autozone do a load test on your charging system- maybe you have low voltage causing issues. If they still act up, check your sending units - the Water temp and oil pressure are cheap enough. If the grounds don't fix it, and the sending units don't, maybe a bad cluster or loose wires in the harness.