I had a similar issue. It ended up being the CEM. I had a forum post about it with more details here.

I'm following up with a final update in case others with a 99 S80 have the same issue. Long story short after replacing the ABS control module the symptoms went away for a few weeks, and then returned again. The repair shop ended up diagnosing and replacing the CEM (central electronic module), which...

I took it to a local import repair shop where their tests seemed to indicate a faulty abs control module was causing the error messages, and probably affecting the Ignition Immobilizer so that it also would not start. I replaced the ABS control module with one from Xemodex. So far so good.

Thanks for the reply. It seems to be shifting smoothly into all 4 gears, though the fluid looks really dirty, so I'm planning on doing a drain, refill, and filter change on the transmission. It's not displaying any codes. I'm still unsure what is going on with the "car starts then turns itself ...

I'm having the exact same or similar issue: "transmission fix soon" message. Then every so often "brake failure stop the car asap", ABS lights, come on and car won't start. Disconnect battery, all messages go away but "transmission fix soon" message stays. After reading...