E38 Transmission Problems (5HP24)

Hi people, I've got an issue with the transmission in a 1995 E38 735iL. It's a 5HP24 model tranny. The car has about 220,000 miles on the clock and runs well for its age. We've never had any issues with the transmission until now. So the problem started one day when I went to drive off from starting the car in the morning, it reversed out of the driveway fine and then when I went to put it in drive and take off it revved to about 1800rpm and then jumped into gear with a bit of a bang and then came up with a message "Trans Failsafe Prog" and the car took off slowly like it was in 3rd or 4th gear. I pulled over, put her back in neutral and revved it a bit. It was still in failsafe mode so I shut the car down, locked and unlocked it started it back up let it idle for a bit to warm up and same thing happened. It takes off fine in reverse gear but when it's in drive, sports mode or in manual mode it revs to 1800rpm then jumps into gear and goes into failsafe mode. I decided to top up the fluid thinking it might be a bit low, seeing as I had to get under the car to top it up, I decided to change it. I drained about 6 to 7qt out but when I topped it up it only took about 3.5qt. I followed the procedure in the ZF manual to top it up (ie. initial fill, run the car with A/C on and shift through gears then final top up) and thought it was strange that it only took half of what I drained. Afterwards it's still doing the same as before only this time there's a whirring/whining noise at low speed. I have a filter on the way but thought in the meantime I'd ask to see if anyone else has had a similar problem or if anyone has an idea of a fix.

Cheers

E38 Transmission Problems (5HP24)