Pocket Hack Master overclockes the PXA270 to 733Mhz-800MHz.
Your problems are likely more software than hardware.
(see here)
Pocket Hack Master (NEW, WM5!) on PocketGear.com
(it overclocked my X5 to 600MHz with no problems)
By the way... Pocket Hack Master tests the speeds BEFORE you can use it.
If it passes, you can use it. It can also underclock it (not sure how much PXA270 can). It also can "auto scale", but the new one seems unstable doing it.
P.S. The app does NOT have to run all the time (sticky mode).