We have a Honeywell RTH8580WF WiFi Thermostat that was installed by
repurposing the G-wire to substitute for the missing C-wire as described
in a Honeywell online video: move the G-wire from the G terminal to the
C terminal at the furnace end, and jumper the G and Y terminals, and
connect the G wire to the C terminal at the thermostat.

Everything worked fine as long as we were trying to heat the premises,
but now that we are trying to cool down, the thermostat indicates "Wait"
for several minutes, then clicks, and the screen goes blank and the
thing "reboots."

The furnace is an ancient Singer, and I can't see a brand name on the
A/C compressor.

The same thing happens if I turn off the breaker for the 240V to the
compressor unit.

Perce