Hey I really need help.

My 99' 9-5 has the 'check transmission' light come on and the car loses accelerating power -this happens perhaps 3 times a month. When this happens, the car will rev but will not accelerate - turning the car off and on fixes it instantly. The mechanic ran it through his computer which just said 'mechanical fault' - I need more clarification. Is this an electrical problem? (seeing as it's fixed upon a restart)

Any help would be appreciated!
Thankyou,
Brent