Why is my F-250 stalling?

I have a 1991 Ford F-250 with the 5.8 engine. I've had trouble with it stalling and being hard or impossible to start. I changed the spark plugs and air filter and it sat in the garage for 2 months because it wouldn't start. I replaced the fuel pressure regulator the other day and got it started. On the way to work, it died. It would start and run for a few seconds but the battery seemed to be dying fast. I finally started it and revved the crap out of it and made it to work. No problems on the way home. I think my alternator may be a bit weak. Could that be what's making it die? I also have a check engine light on but haven't found a free tester that works on a '91 Ford. It seems to run nice and smooth when it's running.

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