First post, so feel free to take it with a grain of salt..
I've had a very similar experience as to what the OP is describing on various rev's of GX. When I was still trying to figure out the differences between the channel wad vs. the forwarder wad I ended up trying out a few different revisions, so here's what I found happens on my system:
-815: Can't get this bugger to work. Black screen every try, regardless of off/on/anti 002 settings.
-686: Worked alright for a while, but only after having to power cycle after usually 2 black screens in a row trying to load a game. Also could not switch from one game to another without a black screen. Sometimes would black screen when changing to systemmenu. Sometimes would black screen when loading Loader GX.
-785: See rev 815
-713: My golden rev so far. Games seem to work on the first go each time. I was using this rev originally then somehow managed to downgrade myself to 686...I'm THAT good!
Each of these revs were tested as forwarders and as channels as well being loaded in HBC, using cIOS38rev13a and 14 (may not have made a difference but hey, nothing ventured, nothing gained).
I'm on 4.2U firmware, system purchased 2nd q. '08. My suggestion for what you're experiencing would be to try out rev 713, and maybe you'll get some positive results. Hope this helps.