BerndP:
Hi All,
Thanks for the replies.
At this stage the timing is not such a great problem, it just seemed odd that identical boards (5 x Pro Mini), just differ so much.
I read thru the other link posted in tim7's reply, and came to the conclusioon that I should check my Circuit again, as I may have erred in my crystal circuit design.
I'll do some more checking and reply again.
What's the tolerance of the capacitors in it? Crystals are usually specified with a few decimals of accuracy but the capacitors might be 1% (or worse). 1% of a minute is significant.
Really though...it's not an 'error'. It's just the way things are.
If you needed to synchronize the LEDs you'd have to connect the devices together, there's no way around it. No amount of fiddling with crystals would solve it.