I live in Phoenix, AZ and have an old Trane heat pump circa 1993. Over the past two weeks, several times the unit has stopped cooling yet the inside and outside units kept running. When this happens, I switch off the unit at the thermostat, wait a while, and switch the unit back on to see if it starts cooling again. If it doesn't start cooling right away, I switch it off, wait a while longer, and try again. Eventually it starts working again, but a couple times it took around 4 hours before the unit started cooling again.
Twice when the unit stopped cooling, I noticed the air handler unit started to hiss and whistle for a short time right before it stopped cooling. I touched the smaller copper line and it was crazy hot--like hot enough to burn my hand. This got me wondering if some kind of thermal cutoff is happening to prevent things from getting overheated.
I recognize that I'm already on borrowed time with such an old unit, but do these symptoms suggest anything in particular?