Check your polar alignment first obviously, I thought I was pretty good till I did my little exercise in rotating long exposures. No almost no guiding required and almost zero drift. Thread here if you're interested. My symptoms were identical to yours.
http://www.iceinspace.com.au/forum/s...d.php?t=151186
The other factor could be balance of the mount/scope/counterweight. Do you rebalance when you change scopes over ?