95 Grand Prix overdrive behaves like neutral

My GP is mint with only 85,000Km. It's powered by a 3100 and geared via 4T60-E.

When running in overdrive (D) the RPMs stay around 3000 at 100Km per hour. The transaxle never shifts to overdrive.

When forcing it out of 3rd, by easing off the accelerator going down a hill, the transaxle behaves as if it's in neutral. Ie: the RPMs drop to idle rate and you can rev the engine, watching the tach needle go up and down.

Dropping it manually into 3rd causes the RPMs to immediately jump up to meet the rate of travel.

My t-fluid is clean and at the right level.

I'm suspecting computer/electrical troubles because there's another issue: the cruise fuse under the hood was missing when I purchased this car just days ago, and replacing it causes the "lights on" chime to activate when turning off the car and exiting. The lights are actually on very dimly too.

I had to pull the fuse again to avoid this. I can live without cruise, but would love to have my overdrive.