Transmission problem

My 2001 Jetta 2.0 engine has a slight slip going into 3rd gear (auto trans), when I first start the car-up it runs good with no slipping. But, when the car warms up after about 3 miles, 3rd gear starts slipping. On the highway, if I drive the car for 35-40 minutes and if I was to slow down to pay a toll it shifts into a lower gear at 60 MPH the RPM go up to 4200 RPMs. When I pull over to the side of the road, turning the key in the off position restart the car the problem goes away Help! Jettajill

Hint, tell us how many miles you have on your car and how often have you changed the transmission fluid and cleaned the filter? Do you have any CEL’s?

I bought the car used 1 year ago, with 113,000 miles; I now have 123,000 miles on it. About 2 weeks ago I was told to change the filter but keep the original trans fluid. After doing that I’m still having the same problem. I don’t have any CEL’s
lights showing up on the dashboard. Also, had a mechanic check it out nothing showing internally. Someone told me it could be the computer?

I have no idea why you would keep the old fluid. That is positively goofy, but its probably based on this old take about the fluid carrying around bits of friction material that actually help the clutches work. The whole idea is a bunch of hooey. What color is the fluid? How does it smell?

Take it to a transmission shop and have them put it on a scanner - most general automotive scanning equipment misses a lot of transmission info including codes (i.e. no check engine light or no standard OBDII codes doesn’t mean no codes). I’d lay some doughnuts on there being codes in there. What it sounds like is your computer is sensing a problem and putting into some kind of limp mode. There are some kinds of codes that may get reset when you shut it down & fire it back up - so the ideal situation is to get it acting up and bring it to a a transmission shop at that time, leaving the engine running (or at least the ignition on).

Ask the transmission shop about the fluid. They’ll likely tell you that you need to replace it (except they’re also likely to be saying not to other since you need a rebuild).

Don’t go to a national chain transmission place - find a local, owner-operated shop.