It's the ignitor!
The ignitor is only usefull for the HPS not the MH, that's actually one of the tests to check if the ignitor in a HPS setup is bad, put a MH bulb in, if it works the igniter is bad if it doesn't it's something else.
Do you still have the fan over the light blowing down towards the plants? In that case I would move it so it sits under the plant and blows up against the light. The lamp can take the heat but the plants should be cooled, not the other way around. :D
Nice build and good luck.