1994 Ford Thunderbird heater blows cold after a few mins

1994 Ford Thunderbird : My car blew a head gasket, just had a valve job, runs great until I turn on the heater which is the electronic climate control variety with digital temperature readout, etc. The heater blows hot air for a few minutes then turns cold. The engine temperature gauge which was in the normal range before turning on the heater begins to increase toward hot. The temperature gauge returns to normal when I turn off the heater. The valve job mechanic and other mechanic's are stumped, can you help?