Why phase reverse switch only works when cold?

That unit uses many relays in various circuits, they become unreliable after a period of time. Most failures usually occur when the unit is hot. If you have knowledge of electronic service, you might be able to locate the bad relay or component with freeze spray. If you don't have service knowledge, I recommend you contact Spectral directly. You could see about a repair facility that has experience with that unit. Some people on this forum may have a suggestion for a service shop if you mention where you are located.  

Bypass the phase switch all together and use the cables to swap phase. No cost fix ay? Never have a phase switch problem again either.. 

The better way is to have a "POT" 0 to 180 degrees out of phase.

Regards