Not long after I bought my 92 wagon I had driving problems. My coolant was really low too. The expansion tank looked as if was up to the level but in fact it was just a dirty scummy coating that made it look that way. With the return pipe going in half way down the rad I only had about half a system full of coolant. I did fit a new thermostat because it was showing just a bit hot but filling up the newly cleaned tank took about 3 pints! No problems since.
Bottom line, clean out your tank.