Help!! Backfiring after re-doing top end

[skiwesser]

Ok the rockers were right.I had put syn oil in for 1'st time when i re-did the heads & stuff, well the oil control ring on #2 is evidently gummed up cause the oil was flowing past so bad the plug was'nt firing very often hence the back firing.However compression is good on all (between 170# & 180#)
I ran some engine flush in it ,went to 10-40 conventional oil and
mostly solved prob. now it only backfires once (loud) either on the 1-2 or 2-3 shift when driving hard 5000-5500 shift point.
What should I try short of a re-build?
I always run cheap bosh supers .99 cent each & change them often, but I put a autolite rapid fire in #2 same as with bosh super,would a "hotter" plug help?
Sorry so long, thanks
Wes