Transmission Shudder - 99 Crown Vic V8

My parents have a 99 Crown Vic V8 that they got from Budget rent a car. It has been a great vehicle with no problems to speak of. it has about 120k on it now and they have been experiencing what feels like a shudder that feels like its in the transmission. It seems to happen when shifting from 2nd to 3rd and sometimes into overdrive. Has anyone experienced this or heard of this?

while it could be something more serious, 2 things to eliminate.
first, these trannys are hard on fluids, causing the fluids to break down, there is an additive, but when was the last time the trans' was flushed?
second, the bolts attaching the pump body to the tran's have a history of backing loose, this causes a reduction in pressure.
hope whatever it is, its simple! good luck

A good flushing(cooler included) as suggested above will probably cure the shudder. If the flush does not cure it, 90% of our shudder is caused by the torque converter. Most of the time, the flush will cure the shudder though