I've just started a trial of PM5 and I'm having similar issues with erratic colour. Mac mini, Mountain Lion, Eizo CG275W.
I wish I could be more specific. I've been trying to pinpoint the issue for the last couple of hours. It's all a matter of saturation. Some images look undersaturated, others oversaturated. I'll be looking at an image in PM's preview, and then when I zoom to 100% the colours change (lose saturation). Or I can be browsing 10 images in PM's preview (same subject, all shot with identical settings) and just one image will be over-saturated. Something very weird seems to be going on.