I think Tim is on the right track, according to my experiences.  The plugged up coils were also a problem for me, they need to be cleaned often.

BTW, I asked several months ago about geothermal heating and cooling, finally changed our units in June, and the house has been uncommonly comfortable the rest of the summer!  I cannot believe the change in this house compared to our old unit, but do not have enough data to be sure of the energy savings.  My first bill included part of the time the old system was still running, and was much lower than the last one, and the temperature was much higher outside.

Pricey things, these Water Furnaces, but I am sure glad we bit the bullet and changed.  I would never advise someone buying a new house to get a conventional unit.  Our main expense was removing ductwork and old units, but I still feel it was the right move, especially in these days of increasing energy costs.

Jansen Brothers in Effingham did a great job, and at much less price than others that bid on the work.