Ahh ha!
I had 4 2gig DIMMS in the upper riser and 2 1 gig DIMMS in the bottom riser.
I removed both 1 gig DIMMS – High on Prores HQ is back!
So it would seem Final Cut really prefers matching DIMMS in all slots? Or perhaps there is a problem with these 1 gig DIMMS in particular.
Any theories? It seems strange that this would only effect Final Cut and nothing else…