car has an intermittent shudder on 1999 Buick Century

Car runs fine around town but when climbing a grade on the interstate, the front shudders for a second or two. It feels like driving over rumble strips. This can happen two or three times until the hill is crested. Sometimes get the same effect when the transmission shifts from 3rd into 4th at 45 mph. Didn't find any trouble codes. Any suggestions? Car has 85,000 miles.

Asked by for the 1999 Buick Century
sounds like the torque converter in the trans is slipping. take it to a trans shop and have it checked.