1989 Ram has fits

my buddy has a 89 dodge ram 100 with the 318 in it and it throws these fits where it won't start... it will crank normal then crank real hard like it's out of time... then crank normal.. back and forth but won't start come back a couple hours later and it'll start right up... he changed the computer thinking this was the culpert and it just did it again the other night... so any ideas? thanks in advance for everyone's help.

First thing he should do is take the battery and starter out, and bring them to Autozone or the like and have them checked. They will check them for free.