I have the same problem on my 90 LX. I have changed the solenoid but it (the problem) has resurfaced after about two months. I was told to get a new solenoid from Ford (about $40.00) but haven't tried this yet. Also, get someone to check the voltage to the solenoid with key turned on and starting then off. Supposedly the voltage will be 12 volts cranking and 0 volts when turned back to off. Did you get any answers yet to your issue? Supposedly their are some solenoids that do not work well but have seen you have tried more than twice to replace them. I think I am going to try the one from Ford dealership next. Let me know if you have found the problem as mine comes and goes but tapping the solenoid with a wrench seems to disengage starter. Some others have said to check wiring for lose connections and make sure the solenoid is grounded well on the firewall. Good luck as this is a nagging problem.