Which encoder(s) have you used with your payload? On a Win XP target, I've had good luck with shikata_ga_nai...maybe in combination with jmp_call_additive. If you know Ruby pretty good, you could code up your own payload module in MSF; using the existing payload modules as examples.
My opinion on the second part of your post would be to take a copy of db_autopwn itself, and hack around with it to get it working reliably for you. Don't forget about asking on the Metasploit mailing list...HDM and the dev team often hang out there. I hang out there myself, and have picked up some good tips & tricks...not knowledgeable enough yet to contribute much myself