99 Intrigue overdrive transmission slips
 

Guys, my brother in law, is not computer savy so im doing this for him.

He has a 99 olds intreg, 3.5ltr motor, that is a wonderfull car, but hes having transmission problems.

The over drive keeps slipping out. No codes are being generated that a mechanic can scan or read.

The result is lower gas millage on long trips.

Any ideas what to check, or do?
Is this a common problem with this car, or transmission?
They did a flush last week, but its still not working properly..

Any help apprecated. Thanks.

How many miles and has the fluid ever been changed before? You mentioned the flush, but was the pan dropped and the filter changed? If the filter is clogged, it might be reducing line pressure for the trans, but it sounds like the trans might just be on its way out. Wouldn't hurt to try changing the filter first if you haven't already.