My John Deere L120 is "old" in years (about 10) but young in hours (165). Recently experienced the "surging" and stalling out problem. Turns out in my case the tip to check the vent hole in the gas cap solved the problem. I knew it was a gas supply issue (not getting enough gas), but was suspecting the fuel pump. So, I started with the basics cleaning the gas cap vent hole, first with a needle (it was really dirty), then blasted carburetor cleaner (from a store bought pressurized can) through it (while off the mower, of course, and held over newspaper on the ground). What came out was amazing (10 years worth of dirt/grease build up). After thinking about it, that really was the problem - as gas level depleted from tank, a vacuum was being formed and fuel system was simply starved. Problem was always worse (happened quicker) when tank was full (not much air in it). As gas level dropped, it would run longer, but always surged and stalled. After standing for a couple of hours, it always started right back up ...... had enough time for gas to "trickle" into fuel line. And, if I just took the gas cap off (which of course allowed air space to equilibrate) and put it back on, it also started back up.
Anyway, I hope everyone else's surging problem are as simply solved as mine was.
