1995 Park Ave shift issue

I have a 1995 Buick Park Avenue that when driven at a speed of 40-50 mph for say a half mile or more will not shift into overdrive. If I am on the open road where I can accelerate at a normal pace up to 55-60 then it shifts fine.

I can fix the non shifting issue while driving down the road by putting the car in neutral and turning off the ignition, then restart and put in drive. The car will then shift fine again. It think it is probably electronic issue. Both the filter and oil are fresh. Thanks.

You will need to have the PCM read for codes if any are stored... The SES light does not always come on for stored codes..

I am wondering if you possibly have a Trans slip code..

You will need to know what if any codes you may have..