I no longer use OS plugs, the last batch of #8s I got were bad. The connection at the center tap lets go after one or two runs, several were DOA! As a test, I pulled some old/used #8 from old engines and they ran great.
I now use K&B 1-L (#7311) in place of OS #8 and the K&B HP (#7300) for high nitro like my OS 46 VX DF on 30%.(after 2 gallons I finally had to replace it, performance and idle were bad, the element was a dull grey.)
http://www.mecoa.com/acc/glowplug/glowplug.htm
Kurt, you may have other problems.
(A)Too much nitro for that engine
(B)The header is too short or wrong pipe for your setup.
(C)The fuel is foaming at speed. Use 1/4" thick Du-Bro's foam rubber UNDER and to isolate the sides of the tank, use in conjunction with an OS bubble less clunk.
http://www3.towerhobbies.com/cgi-bin...0p?&I=OSMG5870 I've had fantastic results with this. Or you can go with the hassle of a bladder tank.