1998 Chevy Tahoe only 2 plugs fouled

Did a FULL tuneup with quality parts, new fuel pump, new fuel injection "spider" unit (the new mfi conversion for the vortec motors), new upper gaskets. When I bought rig few months ago, #3 and #5 cylinder plugs had ash deposits on them. Rest of plugs were fine. Still had a rough idle problem, changed fuel pump, checked plugs. New plugs were fine except #3 and #5 again, so changed all 8 again. Bought the mfi conversion injection unit, idled and ran alot better, but still had a slight miss. Checked plugs. #3 and #5 again ashed. Replaced just those two as rest were good. Drove 250 miles, with the last 50 or so motor ran very rough. Heavy miss, stuttering. Pulled plugs next morning, again #3 and #5 fouled up after only 250 miles. Ran compression test, all cylinders 160-170 except #3 at 120. Did not do wet compression as I have never ever seen any smoke whatsoever at any time, thus assuming that there is no blow by on rings. Pulled head off, head gasket SEEMS ok, can't see any indication that head gasket was the problem. Did find quite a bit of carbon deposits on top of #3 and #5 pistons as well as in the combustion chambers. #1 and #7 look great, really clean. Valves in 3 and 5 do have carbon build up but do not appear burnt or cracked. So my question is, what would cause the plugs to keep fouling in just 3 and 5? Is it possible that head gasket could have been issue, but just can't really tell? Is it possible that maybe valve guide seals are leaking? Is it possible to have the guides leak without actually burning oil? Anything else I should check or look for?