give the connector a push to make sure its making good hookup.
i have to give it a push everytime i fire up my PPC machine otherwise i get a nice hang or full on crash when it initializes the CGX drivers.
once its all warm, everything is fine and it can run for hours/days with no problems.
usual hint somethings amiss is the mouse pointer on the AGA screen piped through the bvision card before the CGX drivers are loaded, is corrupt.
(yes i do have the nuts and bolts that hold the bvision to the ppc card done up :-) )
i take it you've got your PPC card flashed to a reasonably "recent" revision?
have you set up the screen modes in cgxmode? you may want to re-jig them so they are at whatever by whatever at 60hz for your flat panel, instead of the generic modes it loads as standard.
you've got a good version of CGX loaded? i seemed to get problems with the latest latest version, and had to step back a couple till i found one that worked for me.
hope this helps, :-)