What would make a 1990 Ford Aerostar blower fan for heater - vent - ac sometimes run and sometimes not?

I don't know if this will help. I had the air conditioning quit cooling and found a little (under $15) switch plug that needed replacing. (It sits on a small round sliver tank on the left engine side facing the vehicle) It controlled the air conditioner going on or off. It wasn't switching on when it was hot. Replace it in 5 minutes and the air conditioning works now.+