I had a similar problem with my 87 starting. I replaced the starter and it worked for awhile then quit again. It was in the small wire circuit that puts power to the starter solenoid. I was getting less than 6 volts. Run a wire from the battery to the small contact on the starter solenoid and see if it clicks or cranks (no key required).
I ended up putting a relay in the circuit and using the 6 volt starter wire to energize the relay.

87 Raizer, 4.3V6, Mod TH700R4,231CHD, detroit rear and LSD front, bouncy seats, 2" lift and 32x15 BFG MT tires.