Thats pretty cool Jen, longest i've been able to go with my home built fork mount was 1 min or so with no guiding.
found some good explanations here
http://www.cloudynights.com/item.php?item_id=2755
in the section "I watch PHD's graph and display and all looks great but my stars are still oblong", one possibility suggested is differential flex.
another thread with changing phd settings
http://www.iceinspace.com.au/forum/s...ad.php?t=48229
From one user "increase "RA agr" to max 100, and reduce "RA hys" to 5 in order to stop the "Osc-index" going further below "0.19"