I experienced this once, going probably less than 10mph in high range. Felt like it stumbled or bogged down, i thought it may be the gas I had put in it. Since we were on a rough trail and creeping along slow, I put it in low range and never had the issue again. Mine has had the rollover valve mod since day one, but is still on the original plugs and yes I always let it warm up to at least 150 degrees before I kill it even when just moving it around. I haven't had the issue again, at least that I've noticed anyway.
This issue has come up several times in forums and facebook groups and I'd love to find out what the problem is. Things like this bug the hell out of me.
Sent from my SAMSUNG-SM-G930A using Tapatalk