Honeywell T5 WiFi Thermostat Cycles on/off in Heat mode

I installed a Honeywell T5 Tstat and when it calls for heat, it runs for about 6-8 minutes and then shuts off, and then cycles on and off every 2-3 minutes. I have both gas heating (Heil 7000) and electric cooling installed. Cooling uses the air handler of the gas furnace. When I put a meter on R and C terminals at the furnace, I see a voltage drop from 28VAC to about 4VAC just before the heat shuts off. I would like to install a stand-alone transformer to power the Tstat if possible. The wiring to the stat is W,G,Y and R. I had an unused blue wire and was able to use it as the C wire. Is there a way to hook up a stand-alone transformer to just power the thermostat, maybe using the RC and C connectors and removing the R to RC jumper and disconnecting the blue C wire coming from the furnace "C" terminal?

When I change back to my old battery-powered thermostat with W,Y,G and R connected (no C wire), the furnace runs continuously until the setpoint is reached.