1991 Ford Truck Low RPM upon Startup, Major Sputtering at l

Engine Performance problem
1991 Ford Truck Two Wheel Drive Automatic 125000 miles

Hi there,
I have a few issues that I'd like to bring up hopefully you may have an answer for.

Let's start at the beginning.
Sometimes upon startup the engine will cutoff, happens randomly, I have tried it with everything on, nothing on, radio on, heat/ac on, completely random.
the RPM when I first got this trucked stayed anywhere between 1 and 2 upon startup, it now drops to about.5.
Next when driving at lower speeds the engine sputters and knocks - sometimes to the point it will cutoff - I have been using STP fuel injector cleaner every other fill and it is still not running well, this is worse if I try to stay around 30 - 35MPH. At Higher speeds driving I have no issues at all. Also if I'm below a 1/2 tank of gas and it rains, my alternator will turn over but does not start.
I really don't have alot of money to have this professionally checked and I was hoping someone/anyone might have any idea/answer - no matter how minimal it is just to get me started on the right path.

Thank You.