its almost impossible to unlock a pentium 4... it was possible for old time Athlon XP's and celerons since you had access to the circuitry on the chip, but now that big @55 metal heat spreader on top the chip prevents you from unlocking the multipliers , To improve stability you could try better cooling , apply arctic silver thermal paste to the heatsink , extra fans for the case or a bigger copper heatsink... most likely its your chip overheating causing the stability knowing intel these days ..
you should have gotten AMD shinkuu ... value for money.... and built your own instead of getting a dell...