Then I guess I'd rule out the bearings if your engine is still pretty new. I think using your 13-8 or even a 13-6 as I did and your stock muffler without the baffle in it might do the trick.

Also try a new glow plug. You can always stick the original one back in it. I've had OS #8's go bad after a couple of flights, mainly from running too many rpm's from the start. But, you say yours wasen't too lean and the engine wasen't too hot so I don't know what to tell you from here on in.

Ask some of the experts in here, they might know.

I'm just one of many "RCU dummys" in here trying to help you out.