Changed battery and alt on 1997 Grand Prix GTP, now won't stay running?

When a new problem arises, always check the work you just performed. Sounds like you have a loose alternator output cable or field connection or the replacement alternator was defective. It's something to do with the alternator.

Once you disconnect the power/battery, the computer loses it's memory and the timing will change significantly for a few hours or driving time. All of the wear and tear on mechanical parts in the engine is compensated for by the comuter and the timing changes gradually over the many years and miles driven.

This should correct itself in a few days but if not then you need to do a engine diagnostic and have them advise you what is wrong or what codes popped up on the code reader. They will start with a charging system diagnostic and then pull codes if the battery and alternator are good to go.

Good Luck!

This is very common as most cars lack battery cable maintenance.

There is a bad connection on the battery terminals, bad internal battery cable is broken, or the cable to the ground seems to be loose and or has a bad connector.

Sounds more like there is a bad ground from the battery to the engine block and or chassis.