Truck wanting to die while slowing to a stop

Truck drives perfectly while I am driving down the road when I go to come to a stop and have my foot on the brake my lights will dim and then brighten back up then dim again. My rpm's as soon as it gets too 2000 rpm's they drop and then the raise and then drop and then they raise and they will drop and then raise. But, if I keep it above 2000 rpm's everything is fine and every now and then it will actually die while coming to a stop. I put a code reader on it in the only code that came back said thermostat temperature too low for operating temperature. My check engine light has never come on. I am lost on this one.