Volvo: 1999 V70 automatic transmission problem, automatic transmission problem, orange arrow

Question
On several occasions now the car has virtually completely lost power/drive ... the engine is still running fine, but power no longer getting to the wheels (it's almost as if the clutch had been depressed on a manual/stick).  I'm not sure if this is what other people are talking about when they refer to a 'limp-home' mode. Each time it's happened, switch off, wait a minute or two, restart the engine and everything seems normal again except the orange arrow light on the dash is flashing and the check engine light also on.  After the first couple of times the transmission 'engine speed' sensor was changed, hoping that would cure the problem.  After that it seemed fine for several weeks, but then flashing orange arrow came back on; that day, despite the warning light, it made it the 50 miles home as if there was no problem, but the following day during a 2 mile trip it suddenly would barely pull away from a set of traffic.  Stop, wait and 5 mins later is OK again.  Just the flashing orange arrow and check engine.
Also, and I have no idea whether these are related or  completely separate electrical issues but, well before this transmission problem started occurring, we've had strange lights showing on the dash ... usually Brake and ABS warning lights coming on intermittently (have had the brakes looked at several times and there's apparently no problem), but also on a couple of occasions on the freeway have seen the speedo needle suddenly drop to zero and the mileage and trip blank out .... just for 30 secs or so ... with no other apparent problem.  The ABS/Brake warning lights have been longstanding, but the speedo anomaly is just a recent thing which, first time, was probably just before the start of the transmission issues.  As I say, no idea if inter-related.
Thanks, appreciate any advice and suggestions.

Answer
Chris,
Since the car uses the same speed signal from wheels to ABS and to the speedo, I would suspect the problem is in the ABS control unit. Now that being the case have the ABS tested with a new control unit and drive the car for a while and see if that also cures the trans slipping. If some of the speed signals are not getting to where they should the trans may not be getting the speed signal it requires.