I just purchased a 1997 Buick Regal GS with the 3.8 liter supercharged V6 I saved it from being scrapped because the brake lines were bad. The motor runs good it'* throwing some codes I'm trying to Fix working on it slowly. The P1200 code has me stumped, hoping that a tune up corrects the P0303 code.

https://www.gmforum.com/obd-ii-troub.../p1200-292937/

says they are related, something causing improper rich ratio. i think if it was loosing pressure due to a gasket leak or something you would get these codes, though its just an educated guess

A gasket leak? Which gasket? Intake gasket, plenum gasket?

that description seems a little nondescript. think with 303 its likely the wires for injector 3 or the injector is shorted or something. the code is an electrical fault with the injectors.
P1200 - GM Type Powertrain - Manufacturer Controlled DTC - Manufacturer Controlled
Description

Fuel Injector Circuit Conditions
Cause

Engine running, then the PCM detected an incorrect voltage level on one or more injector driver circuits, condition met for 5 seconds

well the vehicle did set for 3 years so this is probably the first of many problems I'll have to overcome

P0303 is a random misfire on #3. Could be coil/wire/plug related. Rarely its the injector, mostly the wire/plug.

could be mouse bit, if it sat. if your lucky you might be crapped up and cleaned with some spray and compressed air.

I would be checking injector #3 circuit with a noid light....it might be causing the P0303....also check the wiring for chafing, on top of the engine, and in the air box where the PCM is...nest could be there...

Well I have never seen this happen before in all the years I've owned General Motors vehicles, but my check engine light has gone off and the the engine miss has completely disappeared. I'm still not getting the world'* best gas mileage, but as I said previously it does need a tune up which will probably help.

I have seen bad plug wires cause this intermitted problem we had one that would only miss under light load going up a hill {very small incline} would buck and you would think.
That the transmission was acting up
and to make it all better no codes would be set
O and the plug wires were BW premium brand about 1 year old