cb500t sparkplugs dying?

rebuilt motor, cb500t, new elec ign, reg/rec, new xs coils wires caps, plugs, after riding for a few days, park it at night, then the next start, will only fire on one cyl, can change sides and see if that works, but no, the other side then fires, so the plugs themselves are dying, what would cause a spark plug a week old to just quit? this is a frequent and only problem with the bike...

What color are both plugs?

what mfg plugs are you using? do not use champions!! i think they take ngk b8es you can go to 7's , but more than likely you are running rich and fouling plugs (black -sooty plug) or weak ignition/ battery ..... only foul on one side?? ck float level first...blue or black smoke from exhaust on that side? the more info the better
joe@ vcycle