Popping thru intake

[SheStang87]

I am having problems with my '87 GT with a stock 302. She recently began to "pop" thru the intake! This doesn't occur all the time, when ever she feels like it I guess. This usually occurs at a low RPM range in a higher gear, say . . below 1500 rpm's in third gear. I have not the slightest idea what to look for.
I pulled my spark plugs and checked them out . . ok, I changed the coil, checked the timing, checked fuel pressure . . . all my lines, the compression . . .and she still does it! I can run her down the 1/4 mile and not a thing happens, but coming down the return road she starts to "pop" again . . but only every other run or so.
I would greatly appeciate any ideas or suggestions that anyone might have.
Thanks a lot,
Christina