The voltage should not "collapse" under load. Measure at the converter output terminals not at the load, allowing for the way the load drags down the voltage at the converter's end of the wires. It should be obvious if the converter can't hold its voltage up.
If the voltage holds up with the non-MH loads but not with MH loads then the converter is ok and you can find the unusual (short?) load in the MH that is causing all this by elimination (one fuse at a time)
Of course if it was a wet lamp before it might have dried out by now so you have to wait till the next time it gets wet. :)