91 GTO auto strange fault

Hi all, just after a bit of info regarding my new purchase, I’ve replaced the rear brake lines and just nipped the car around to the end of road, as not driven it since due to no brakes.
The gears at first seemed to select no problem, but after a few mins started slipping, I managed to switch to L,2 then D again to maintain drive. Since returning the car idles at 2.5k then once a gear is selected drops down to 1.5k.
Is any one familiar with such a fault?
Before driving the car idled around 900rpm so it seems to ECU or something is causing the revs to be high.
Any help appreciated

I managed to sort this fault out,

The revs were due to the Idle Control Valve being faulty, manually adjusted it and left unplugged until i can get one ordered.

Secondly the dropping out of gears was down to the OD button activated, the previous owner mentioned the OD was faulty.
I’m now on the hunt for the end clutch kit.,

Cheers