The thermostats is used for both gas and electric.
If the unit cools on electric the problem is in the gas section. You say the flew has been cleaned. There should be a tubulator in the flew to cause the flame to wash against the sides of the chimney. Is that still there?
Cool the unit down on electric to the lowest setting, a day or more. Once cooled have someone put it on gas while you watch the flame and then have them set it to a colder setting. You should hear the flame sound change as the thermostat calls for cooling. If not the control board or the gas valve may be the issue. Put a volt meter across the wires leading to the gas valve and do the raise thermostat test. If you see voltage as the thermostat goes up the control board is good and the gas valve may not be responding.
WBCCI 12156 AIR 3144 WACHUNG TAC NJ6
2004 Excursion 4x4
1991 34 ft. Excella +220,000 miles, new laminated flooring, new upholstery, new 3200 lbs axles