22 year overclocker here. I concur with Nap's assertion.
As for you lowering the CPU clock on your own, yes you can just do that BUT, unless you balance the other hardware,(mostly RAM speed, and other hardware factors) with your current lower clock you may still experience CTD's.
If your BIOS has a reset to default setting easily settable by you, then thats prob the way to go and find another overclock guy.
Basic pretty good overclock tutorials and other info about the subject can be found at tomshardware.com. Doing it yourself is prob best as it's not hard and what overclock you end up will heavily depend on the hardware not the skill of the overclocker for the type of overclocking you want to do.