90LS4

I just recently bought a 90 LS400, since I have had it, I haven't been able to keep it running for longer than a week. I drove it home about a week ago and I had to stop at autozone and get it towed. When I turned it off the starter kept spinning and started to smoke. I replaced the starter, and for a few days it started right up. About 3 days after replacing the starter, it burned out the motor. What could this be???