Re: 1990 Grand Marquis Trouble or not?
I'd change out the transmission fluid. 10 years is a long time to go without a change, regardless of mileage. If anything else, the old fluid tarnished and left a film over the gears in the tranny (not good). Same with the engine oil if you haven't done so already. Drive around in first/2nd gear and keep the rpms up (after oil change and new spark plugs!) it may require another tranny service within a few thousand miles just to make sure all the crap is out, if the transmission still misses shifts then I'm sure it will require a rebuild.
'00 Durango R/T 360ci 290hp (modded); 138,500m
'06 Pontiac G6 GT 3.5L 220hp; 44,000m
'12 Chrysler 200 Limited 3.6L 283hp; 13,000m
'99 Taurus 3.0L 2V Vulcan 145hp; 154,300m - Traded
Amsoil in all vehicles!