Hey dude, as someone with some electrical experience it really sounds like the HPS is trying to draw too much for the ballast to take, which sounds kind of weird I know. You may find the ballast is not suitable for HPS for some reason (or faulty perhaps?) which may explain why MH works over HPS.. There is also the question of contactors I believe.. I am still knew to horticulture stuff really, so I won't drone on too much. Again, it sounds like a problem with the ballast,

PS: yeah dude, that flickering sounds intrinsic to what I'm talking about.. meh could be wrong though!

Good luck dude,
Denial