Truck turns over fine but won't catch like it isn't getting gas?

Truck has 149,000 mi, for past few months, Alt. Gauge needle has been occasionally fluttering and at a light sometimes would drop below halfway. I figured alternator was coming soon. The other day the truck stumbled leaving a light all gauge needles dropped like a lack of power. Truck turns over fine but won't catch like it isn't getting gas. And if is does start, it is only for a second and will barely run. It acts like in the old days when you blew an alternator and were running on the battery and the battery was just about out of juice. I put a new alternator in and still have the same issue. I am concerned now that it is the fuel pump but before I go through the hassle of dropping the tank I would like to know if I am on the right or wrong path. The dude at autozone seems to think it is a gauge cluster for some reason. That didn't make sense to me. A couple of years ago, someone tried to steal the truck and when I had a new ignition put in, the security light stays on. I haven't had the computer reset and don't know if that has anything to do with it. Money is tight and I need to do as much as I can myself. I live in Phoenix and the temps can be extreme on vehicle components (supposed to be 108 today). Hard on the shadetree mechanic too. LOL. I'm sure I am due for a fuel pump too but I can't afford to do the pump and still not have it run.