Any idea which Thumpr Cam is in your car? The entry level flat tappet SBC cam is a very nice cam, but the upper level Thumpr's have a tendency to foul plugs with their short LSA and long duration. They sound great, and perform well, but are a pretty rich spec for a street cam.
I have a mid level Thumpr in my BBC, and it never fouls plugs at the drags, or cruising at higher rpm's, but if I start and stop it a lot to move cars around it will quickly foul a plug or two.