As in, if it's only stable at say 2.5GHz or lower, then I'd start to try and do tests that proved which component was restricting the clock.
My guess would be that its the cpu, but I'm sure there would be tests you can do that will stress each component (cpu, mobo, ram) one-by-one and work out which one is the "weak link".
I'd try to find the highest stable clock first - if its something fairly reasonable like 2.8GHz-3GHz then I might personally be tempted to just accept it and leave things at that. But if it can't even get that high, then I'd maybe spend some time running tests to try and prove where the problem lies.