I have a 1982 w/350 cross fire, It starts just fine but as soon as the engine gets up to running temp. it shuts down, it will start back up and run for 20 - 30 seconds and shut back down. this continues with the in between running times getting shorter as the engine gets hotter. I have done basic checks for loose wires loose grounds.. ect...... I am not sure where to start this troubleshooting , any Ideas , Please help