where is the torque converter solenoid on 1992 Mercury Grand Marquis

I would check the transmission fluid first. It might be low. Warm up the car first, then check on a level surface with the engine on and in part. Is it leaking? When was the last time the fluid and filter we're changed?

The torque converter is only engaged when the car is fully warmed up, and in 4th gear above a certain speed, say 55 mph on the freeway and not under heavy acceleration. The solenoid is inside the transmission oil pan. That particular solenoid really would't be capable of producing that condition as it isn't used to aid in acceleration. However, you ARE describing what happens when the Ford E4OD transmission starts to fail, usually shows up as it won't go from 2nd to 3rd correctly, or 4th gear gets flakey, then stops working, in addition if it loses 2-3 shift it will never make it into 4th gear either. You can check the fluid as stated above, could be low to the point that it can no longer engage the internal hydraulics. If it's not low/leaking, the trans is just shot, it was a fairly common occurance. My guess is the fluid was never changed, and has 26+ years of clutch material, deteriorated o-rings and varnish build up contaminated trans fluid sloshing thru a known weak unit on heavy rwd car.