Glow plug light comes on when hot

[choll]

Since glow plugs seem to be the word of the day lets try this. My wait light does not come on at all when I first turn the key. I replaced the glow plug circut card with a known good one. I get the thud of the relay working but still no light. on occasion it comes on while driving down the road, this sometimes comes on after driving hundreds of miles. any ideas??