What causes problems with the fly wheel and starter to reoccur on my 1988 Chevy pickup 1500 since both have been replaced?

is the problem,that the flywheel teeth are being chewed out and same on starter.

or is the problem that starter and flywheel do not mesh properly when engaged?

if either of thses is the problem,this means that the gear on the starter is not matched to the teeth on the flywheel.

other problem that could be causing this is none or incorrect shims used when installing starter motor.

to fix this, you need to make a mark on the flywheel(use white out or something like that),and count the number of teeth.then do same on the starter motor.then find out from a drive line specialist,how many teeth shoud be on starter motor to match the flywheel.they can also tell you the shims to be used when installing starter motor to get proper clearance.