If you haven't already take the carb off clean the base and seal it with some sealent and if that does not help It may be time to replace the front bearing. I had the same problem when I first started running 8th four years ago, would run fine for about 2 minutes then it would load up in the infield and clear out on the straight. Check out Boca Bearing's MX Ceramic bearings. I own two C6 BBT7 and they run consistent every time I start them up with no problems. What plug and fuel you running?