Shower cycling hot and cold! Pests in heater vents??

I have had the same problem. has to do with the pump pulsing on and off and the pressures changing.

How I fixed this.
when I set the shower temp.
Turn hot water full on and then put the cold water on until you get the desired temp. If the pump starts pulsing before you get it adjusted.. you may have to adjust the pump pressure switch (this is what I had to do) so that it stops pulsating.