I was having this problem also when it would (or was supposed to) transition from wifi back to 3G but wouldn't without a reboot (power off/power on). Sent an email to HTC about it and they recommend doing a hard reset. Boot into the recovery then go to "clear storage" and do it.
BE ADVISED, i had to reinstall all apps email accounts etc. and lost all app data doing this but it seems to have worked...haven't had the problem since and that was 2 weeks ago.
p.s. i'm also rooted via unrevoked running stock 2.2 and no fancy stuff.