I have a Webra Speed 50 on my Sig Something Extra. I can't get it to quit sagging. I also coughs some. I sent it in to Webra. They put in new front bearing and piston and sleeve. It runs better but still cuts out. Just can't depend on it. I am running Sig 10% and a OS 8 glow plug. Could it be something about my tank? Any ideas?

Thannks