vertualbernie, may be too late to help (or you may have tried this anyway). When switching from heat to air and vice versa, did you go around and change all the vents? Hot air rises, so in winter you want all the vents on lower floors open and all the vents on the top floor closed. Cold air sinks, so in summer you want to close all the vents on the lower floors and open the vents on the top floor. We have to do that or we would have the same problem you had.