I have a 1988 with a 351 and I hop in and it drives great but after about 20 mins the temp gauge drops and it starts missing has new plugs.. any help is appreciated greatly.

 

should be efi, have you pulled any codes? has new plugs, what about wires, cap and rotor? have the fuel pressure checked. may need to replace fuel filter or pump worst case. i dont understand why the temp gauge drops unless you have either an electrical issue or the Tstat is sticking and then finally opens when temps get real high. Probably should consider getting a real temp gauge installed so you know exactly how warm its getting. might consider a new tstat (195*) and radiator flush after you get the gremlins worked out of course.

 

First thing I'd do is add some extra grounds between the motor - usually the throttle cable anchor plate rear mounting bolt and the passenger side hood hinge bolt. Soldered lugs that fit the fasteners. Never-Sieze the bolts. Maybe an extra ground between the bat (-) and an alternator mounting bolt.

This will allow the ECM and the engine (sensors) to communicate better. The old OEM grounds are getting crevice corrosion and teh bolts are getting rusty... You may see the temp gage react better. You will see a difference in cold start and warm-up regimen...

Once that done, I'd replace the T-Stat. How are the coolant hoses to the throttle body? Any fluids dripping anywhere?

Is it bucking, pinging, knocking or doing anything else unusual? Sometimes the knock sensor will hear engine ping that you can't (unless listening carefully) and the ECM will pull timing out until it starts bucking - has happened to me...

New plug wires carefully routed and reset timing per factory setting and it's all good again

I like Moroso spiral core wires, but they are other good ones