89 K1500 starting problem

I have a 1989 K1500 with the 350 V8. My mechanic just replaced the intake manifold gasket as it was leaking oil. I started it today and it started fine. I drove it about 10 miles and stopped for gas, and it barely restarted, like the battery was suddenly weak. Fearing it would not restart, when I was loading an item at the store, I left it running. When I put it into gear it stalled and I had to restart it. It was slow to start again. It ran fine on the way home, held idle at stop lights ok. I put it into reverse to back into the drive and it almost stalled. I gave it gas to keep it running. I plan to replace the battery, but I know my luck and it probably won't be that easy. What else should I check that the mechanic might have messed up during the manifold gasket change? The neg cable appeared secure to the manifold. I'm not sure what else to look for, and if I take it back to the shop and the problem doesn't immediately appear, I'm sure they will send me packing without doing anything. The fuel pump was replaced on the vehicle about a year ago. No other work has been done.

How old is the battery? Get the battery load tested at a auto parts store they'll do it for free.
AS far the stalling goes I'd look for a vaccum leak on the manifold. Spray some carb cleaner along the gasket while its running being careful along the exhaust manifold. Carb cleaner is flammable.

The battery is older. The odd part is that if you leave it sit for a while, the engine turns fine. If you try to start it while it is warm, it turns very slow like a bad battery. It is almost like something is draining it down while it runs...I will check the manifold.

simple thougt but is the alternator hooked up? unless you are running the lights it will run along time without the alternator. simple check is to start up and pull the positive cable from the battery. if it keeps running the alt is working.

timing could be an issue as the distributor was out for the gasket change.
wouldnt hurt to take it back and show them what it is doing.
the symtoms you talk about lead me in that direction

Tried the disconnecting the positive cable test. The truck stayed running. I then turned on the headlights with the cable still disconnected and it immediately stalled out. What does this tell me? I tried the same test on another vehicle and it didnt stall with the with the cable off and lights turned on. In any event, the wires at the alternator are attached and secure. I haven't tried the carb cleaner test yet as it is 31 degrees and raining here, not pleasant work conditions...

As mentioned, do a load test on the battery. If that checks good replace the battery ground cable.

sounds like the voltage regulator in the alternator is not putting out enough juice

I replaced the negative cable. No improvement. Still motor turns fine when cold, but turns slow to restart when warmed up. Since it turns fine when cold, I'm guessing the battery is fine. Should I pull the alternator and have it tested? Idle issue is sporadic. It sounds fine cold, but the idle surges sometimes when warmed up. Today I ran it and the idle seemed fine. The day before it was surging up and down. I need to get some carb cleaner to test the manifold gasket.