The surging sounds likes your fuel system/carb needs a cleaning. Take your aircleaner housing off all the way, with the two bolts that hold the housing on to the carb. Spray the carb down with some cleaner.
Also put some Seafoam additive in your gas tank (follow directions on can). Put all back together and run for awhile. If still surging after that you may have a leak in the gasket on the bottom of the carb.
The engine shut down could be a number of things. Since it is definitely heat-related, it could be one of the coils getting warm and failing too. That ones a little harder to diagnose.
Thanks for the advice, I'll try both. Finding Seafoam in this neck of the woods is a challenge, seems only 1 car parts store carries it and that off the beaten track.
It seems that the majority of responses posted on the site come from you. That leads me to think that you might be a parts dealer, as well as a fount of helpful information--is that right? And if so can I order an oil filter for a 6800N from you? Thanks.
And thanks, again, for the advice, I'll follow it. Tom
No, I'm no dealer, just someone trying to get more info out on these mowers, as there is basically none.
I had to pick one up as a backup to my JD 318 and did a lot of research. All tractors are basically the same, with some minor differences. These tractors probably have the least amount of info out there, but you can still get service manuals for them too.
A dealer actually runs this site and you can order parts from them... www.yamahagenuineparts.com
or www.canns-bilco.com or
http://www.powersportspro.com/pages/parts/viewbybrand/2/Yamaha.aspx or you can also substitute a NAPA Gold oil filter #1635.
Good luck with the surging problem. Doesn't have to be Seafoam, basically any gas additive that cleans up deposits will do. I have just had good luck with Seafoam, and have been using it for 25 years.