96 4l60e Problems

I have a 1996 Olds Bravada. Recently, I've been having transmission problems. It is about 18 miles (25 minutes) to work, about 12 miles in to the trip there is a stop sign. Up until this stop sign everything runs fine (everyday), after the stop sign I lose overdrive. Three miles later, another stop sign, this time it starts off in 2nd or 3rd (not sure). Now for something different, since there are hills in the area...I get the vehicle up past 55mph, put it in neutral, and turn the vehicle off for 10 sec., after I restart it i get overdrive back. That is until it goes below 35mph and then the same problems come back. I've noticed that other post have experienced stalling and other problems, but I haven't experienced anything but this.

Any help will be appreciated,
Thanks

It is droping into fail safe mode which is locking into 2 gear mode to help you get home.

Gewt it on a good transmission capable scanner and read codes and read out transmission data.
Post back any codes.
Any check engine lights?
Speedometer working?
Fluid and fluid level good?
What is the mileage?
Ever been serviced?
Any wild killer transmission flushes or fluid type changes?

The default gear for a 4L60e is 3rd gear not 2nd.
2 service bulletins come to mind when reading your description. So are you using a K&N air filter? If so, it could be overoiled and could have comtaminated the MAF sensor which is known to cause transmission shift concerns.
The second bulletin involves aftermarket electronic device such as a remote starter or alarm system incorrectly installed or a bad ignition switch causing a loss of power to the transmission solenoid power supply circuit.

I was thinking of electrical problems on this too. A failure of electrical will default to third gear. Like said before, you need a scanner, preferrably tech 2 and manually check the shifts with the on-board scanner. This will let you know if it is in fact an internal tranny problem, or something electrical (PCM, connections, etc.)