This is quite common. Your tranny is old, and the clutches are worn. Old fluid has become thick with wear and tear and thus compensates for worn clutches. New fluid is too thin, and worn clutches tend to slip during shifting.
The alternate explanation would be that the internal parts of the transmission, the oil passages and oil pumping mechanisms, have become too large from pumping around thicker fluid, to accommodate new thinner fluid. This is a form of damage.
The mechanical reason for this is still out there, but basically, the problem manifests when you change to new fluid, on an old transmission.
The solution is to replace your oil with thicker but new tranny oil. Either royal purple or valvoline has got a product called high mileage transmission oil. Use that. And after that, please do report your results back here.
I'm sorry, I know that this is going to be an expensive experiment, but someone here had the same problem, did the above, and fixed it that way. Please give that a shot. Your alternative would be to get a used tranny or rebuild your existing one, both of which are expensive affairs. [Since you've got a near classic car, you might want to consider a rebuild if you intend to keep her for the long term. ]
For good measure, before you change the oil, do the following :
1. Clean out your throttle position sensor. Use contact cleaner on the sensor and its socket, use a toothbrush to clean the contacts, apply dieletric grease and fix it back.
2. Delete all your error codes. Unclamp both your main and transmission ecu. Wait a few minutes, then reclamp them back. Start the car and let it idle with no throttle feathering, for around 10 minutes. Then go for a drive.
3. If the above two methods executed in sequence fail, then you've gotta try the high mileage tranny oil. You can keep your existing oil filter (assuming you changed it together with the new dex3 oil you just put in). Transmission oils are compatible with each other, so don't worry about flushing etc.
Please keep us updated about your progress. Thanks.
Last edited by robertobaggio20; 12-02-2012 at 08:22 PM.