OLD RCER
I just built a stamp or PBF and mine did the same thing when flying slow in head wind. I have a O.S 40 FP on mine with a 10x6 prop.I also use the side pieces like a few PBF flyers have used. I have found that putting a 24" piece of 5/32 steel rod in the 3rd flute on the coroplast on the leading edge fixes the problem. I can now fly full throttle and kick out some serious aerobatics with it. Plus the extra weight gives a little more up front to help get the CG right.
Hope this helps you out
Jason