88 crown vic transmission problem

I have an 88 Crown Vic (5.0 L) that is starting to have a problem shifting into OD. :banghead: It shifts smoothly on the low end. After the car has been driven 10 minutes or so it will finnally shift into OD at about 45-50 Mph but it acts like it really doesn't want to. The fluid level was a little low but still bright red so I added some to get it to just below the hash mark. BTW, I recently had the air conditioner serviced (checked for leaks and added coolant), but I don't think there should be any correlation between that work and the transmission problem. Any ideas about where to start and what to watch out for if I have to take it to the shop? Thanks in advance for any help!:tongue:

Is the coolant temp sensor and the TPS within range? Both will have an adverse effect on shifting. Also check the grommet on the kick down linkage.