I think Robo is onto something... you need to get it dynoed and post up the A/F ratios etc... my bet would be its just running extremely rich and blowing out your spark... remember the closer you gap your plugs the weaker the spark gets (may help you fix the problem in the short term tho)
other than that if you reckon it is losing boost could you just have a boost leak somewhere that only become apparent when you go over 7psi?