Sounds like you're not using overclocker orientated boards. (P43 is the cheap version of the P45, and Intel boards don't rank highly with overclocking).
Go with boards that rate highly on overclocking benchmarks.
Also I think your E5300 could've used a bit more voltage
If you can't manually set it, that blows. Can be lots of other things though, there is a lot more than just Vcore.
I'd expect you'd be able to get well over 3Ghz without cold start issues on the i7 920, I suspect your intel board is keeping every voltage as low as it can.