gear box trouble

History :
December 2001 - GearBox crapped out
January 2002 Fixed, (on the way home from the garge, broke again
fixed again in january
2weeks after being fixed, i was driving at 60kmh and it went up to 160kmh, but didnt increase in road speed at all

to the point
today, got the car back form getting the rear wheel bearings replaced, driving along and the same happened, the speedo jumped up to 160Kmh, when i was doin 80
then 1 minute later i was doing 60, and it went up to 80, but still didnt increase in road speed,
trans feels VERY hot around the gear shifter, and when it goes up to 160(im ony going 60) the engine revs up real high pitch, (or maybe the gearbox)
what is the reason for this