It sounds like you have stripped the gear teeth on the left side of the seat. This is usually caused when reclining the seat w/weight against the seat. and not pulling the lever up far enough. The same thing happened to me in my '90. If there is enough material left you might be able to smooth out the teeth, but it is unlikely. You will need to find a new gear set for it from a junk seat for a permanent fix. A quick short term fix(in my case it was 8yrs) position the seat in a comfortable position and drill a hole and use a bolt to lock the seat into place.