Say you're out cruising at 45mph, then you let off the gas pedal and the car just lurches and clunks. Sound familiar? Well, we're happy to report that it's probably not a serious problem. It's most likely the universal joints on the driveshaft. We can honestly say that we've never been in a Cougar or Thunderbird that didn't have this phenomenon. Most people assume it's in the transmission or rear end but tend to forget about the lonely little driveshaft. But unless the car has over 150,000 miles or so, don't rush out to have new U-joints put in. They're good for at least that long and are very low maintenance items. If you still wish to have the U-joints replaced, you're in for a welcome surprise: it's dirt cheap. Virtually any machine shop and transmission place (and sometimes a good parts store) can do the work for you, for well under $100 US.
We've also been told by mechanics that it could be something in the tailshaft of the transmission. While this would be an impractical (and probably expensive) problem to fix by itself, it's something worth mentioning if you ever have to get the transmission rebuilt or replaced.