In o'drive, I doubt you would get an engine surge large enough to affect your tach to any great degree. The gearing of those puppies is such that there would have to be one helll of a mph change to make your tach change significantly.
My bet is that your transmission is either slipping or "hunting" from 3rd to o'drive and back. Probably the former though. Is your trans fluid up to level it is supposed to be at idle? What color is it? What does it smell like? Burned smell, etc.
Does your tach change + and - in response to assumed engine speed?
Give us more info.
2001 SSEi- black; stock 92K
1995 Vette; Competition Yellow; stock; 38K