[tirwin]

Could this issue have another underlying problem like a vac leak? You mentioned backfires so that's what got me thinking. What is your fuel system? Stock CIS?

I might also try backing off the plug gap. Start at .028" and if the problem goes away then increase by 0.005" - .033", .038", etc. - until it comes back and then back off to the previous.

What kind of plugs are you using?