Not over agitation but uneven agitation and development. Rotational agitation went out with the arc (except in processors such as a JOBO. You don't say what make of tank you are using or what developer. If it is one that is completely watertight then inversion agitation is the way to go.
This involves turning the tank upside down several times. For instance a normal fine grain developer needs 10 inversions at the start of the processing time and then left until the next minute is up then a further 5 inversions every minute after that until the end of the development time.
If you are using something like Rodinal this requires 30 seconds inversion agitation at the start of the time, then one inversion every 30 seconds.
Give us some more information and we may be able to help more.