Wont Stay In Overdrive

I have a 95 Olds Silhouette Van 3800 with 66,000 miles I got from my father who cant drive any longer. It runs great but has one issue. It seems like after you drive it for 10-20 miles or so It wont stay in overdrive. When going down the freeway at 70mph the rpms are about 2000-2200 or so. Then after about 10-20 miles the od kicks off and the rpms stay around 3000-3200 rpm. It seems like if I pull into a parking lot for a while (say go to the store) come back out its fine again for about 10-20 miles then same thing. It seems to shift fine and no other problems except every now and then it will die. Any idea? Thanks X

One other thing....are there additives that I could use to fix this. I think its called green sea foam? X

First thing, did you check the fluid level? If that's OK, the very next thing I would do is a transmission fluid and filter change (not a flush). You can try some sea foam trans-x (I think that's what it is called). It's made by the same company that makes sea foam. It's good stuff, but you don't want to leave it in there very long. Put it in, drive for a few miles, then pull the pan, change the filter and put new fluid in.

Yes, a known issue. This can very easily be caused by overtemperature of the transmission, There is a thermal temperature switch in the transmisison (in later models, a temp sensor linked to the computer) that switches the transmission out form TCC and 4th to lower the stress on clutches. The transmision is cooled by a serpentine loop within one or even both of the main radiator side tanks. It happens these loops corrode and invade the cooling system with trans fluid. Then the oil lines from the transnission might have been disconnected and U turned without going in the cooling. This rapidly runs the transmission too hot and what you describe happens. Units with a towing package use an external additional transmission cooler in front of radiators. Check for accumulated dirt. SUch unit can also be added to standard models. I do not know if the external cooler is standard in hot climate areas.
My transmisison cooling loop did contaminate my coolant last fall;
http://www.automotiveforums.com/vbulletin/showthread.php?t=965632
I was 150 miles from home and I bypassed the loop. Soon after I left returning home, maybe 50 miles in the trip (is it cooler than Vegas in my place) 4th and TCC became inoperative and I returned home in 3rd. THen after that I added an external cooler. All returned to normal.
IF you do not have an external cooler, add one. It might compensate for a temp switch that has become too sensitive.
See what you can verify about the car history and come back . I also replied also in the APV forum ; this info is relevant of Transport/APV/ Silhouette 4T60 but I will continue replying here to respect forum rules.

Ok thanks for the help.

I checked the fluid level and added about a pint. Smells ok and the color is still pretty red with a hint of brown.

I just drove from San Diego back to Vegas yesterday. It stayed in O/d for about 1/2 the trip. What I mean is....it stayed in O/d for about 100 miles and then popped out and would not go back to O/d. It also died once while driving about the time it did that. Anyway I stopped for a while at a Mc Donald's then started up again and it stayed in O/d for about another 150 miles or so then came out again and was out of O/d the rest of the trip homes. After I got home and unloaded the van, I drove around the rest of the night with no problems.

As far as history the van was my fathers. It has 66k orig miles. I don't know about the bypass but I will check today and post up what I find.

I was doing some reading on the net about this and there is some information about a TCC solenoid going bad? I don't know if that is what this is or not.

I did take it to Checker Auto Parts last night and had it scanned. No codes found.

Thanks X

Loss of TCC only would not rev it at 3000-3200 at 70mph at steady speed on flat road. YOu can check that easily by shifting from OD to D when it behaves as you describe. IF it is already in 3rd, you would see no difference.