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Yes, thanks for reply. I've always made sure my autofocus is on, but your other suggestion about cleaning the focus lamp - I never considered that before! Anyway, I used a microfibre cloth and then tested my flash on both my cameras. The results were good, although there was still a very slight struggle with my 5D2 focusing. Now, my tests were in a controlled situation (my dark laundry room). My real test won't come into play until my next big event shoot in a few days.
Thanks again. I'm still open to other suggestions from others - I've researched this problem to death!