first of all, agp8x will yield no real benefit over 4x, so stick with 4x to solve your stability problems
second, i doubt it uses enough power to cause instability by itself. i don't knwo for sure that it draws more power than agp4x, but assuming it DOES, i don't think it's very likely that just that little jump in power consumption pushes your power supply over the edge. if that were true i would think you'd see instability when running your system really hard even on 4x
i think it's more likely that your motherboard doesn't do a very good job with its 8x support.
again, if you're stable on 4x, just stick with that
if you weren't stable on 4x i'd say get a new power supply
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