It could be that your expansion tank cap is faulty and letting coolant leak
out into your overflow tank and that this is causing a slight overheating
(boiling coolant will pressurize things.  I would replace the cap, bleed the
system, and then see if things are better.  Also you might want to check that
your coolant mixture is on the money (buy one of those floaty tester things
from FLAPS so that you can be sure that this isn't causing the problem).  
Hope this helps.

Thanks,
Ken Wilford
John 3:16
www.vanagain.com
Phone: (856)-765-1583
Fax: (856)-327-2242