Default MT6577 1GHZ overclock to 1,68 - phone crash ofter offten ( prestigio 4500 duo )

How much should i overclock this CPU to not make phone crash (Prestigio 4500)?
circa 1,4 phone works good, but hot, if i overclock to 1,68 or 1,7x phone crash after boot, i use to OC modified kernel from china phones rom (bedove or something like that) ..
Guys, what method to OC is better...
I change voltage etc, but cant make it stable..