97 start up rpm jump
So I'm looking at this 97 powerstroke crew cab 2wd auto with 165k miles. It's in decent shape and the seller is not asking very much for it at all. When I showed up, he had it running and warmed up which I don't like cause it makes me think he's hiding something. Everything seems fine with it. It runs well and drives well so I'm mentally ready to buy and I ask him if I can shut it off and start it up. He says go ahead but the batteries aren't very good so I'm not sure if it will start. You can tell the batteries aren't very strong, but it starts fairly quickly. Immediately there is a type of lope but its not a good sounding lope. It will go from 350 to 800 rpms and bounce back and forth for a while and in order to even it out, you have to hold the accelerator for a few seconds. The owner said this never happened to him (I think he was lying) and he said its probably because of the weak batteries (is this possible since once it is started it should be running in its own?). Am I looking at bad injectors, a weak hpop, a bad idm or cps, or is it possible that a weak battery means weak glow plugs? The battery gauge on the dash jumps with the rpms.
I would appreciate any help you guys can offer.