Janet -

The overdrive is likely the problem. But, here's a check item: Watch the speedometer when the problem occurs. The drive gear is at the output end of the transmission, and if the drive shaft turns you get a reading. If the clutch slips or the o/d isn't right, the reading may be low or none.

Also, sound from the driveline transmits into the car body and can seem to come from anywhere. A clunk in the rear may not actually be coming from there. My usual check it to drive next to a smooth wall, like along an alley behind a strip center or big store. Driver side 3 ft from wall, window down, and then make it make the noise. You can ofter tell which end of the car is the noise source. Works well for checking U-joints, too.

Good Luck,

Bob

:>)