mac pro 3.1 no display fans on high

no display on cold startup, heat inside 3 mins with hair dryer then starts, crashes after (variable) half an hour, screen goes black fans go to max have reset various pram ect. changed graphics card, lights in ram bay are one amber (stby?) 3 green. if I then heat the logic board agin for a few minutes it restarts ok but eventually crashes as the fans cool the inside. Any pointers much appreciated