I recently wondered something similar. First, quick question. How come you're using PHD for guiding when you have Maxim already?
In my case, turned out to be poor seeing combined with too short exposures on guide cam. I lengthened the guide cam exposures to 5 secs to average out the seeing a bit more and that made big improvement. Also set a delay after exposure of 1 sec to ave out seeing effects, and tweaked "min move" to better suit my imaging and guiding gear.
Good link on tweaking guiding settings:
http://www.ccdware.com/resources/autoguidercalcv4.cfm