2004 chevy avalanche

gas gauge don’t work, heater only works when you drive at least 70 for about 5 mins then stops when you slow down, and it says engine coolant hot.. thermostat was replaced and heater core flushed. dash cluster was changed . after that everything worked fright for a day or 2 then it all stopped working again.. i don’t even think the mechanics know whats going on.. please help its freezing here.