I have a Hustler Super z with a 27 HP Kohler Command Twin CV750 engine. It has recently started to lose RPM's intermittently. Doesn't matter if under load or not. I have replace plugs, carb, fuel pump, in-line filter, in tank screens and fuel lines. It always starts, and will initially run fine, sometimes for hours, then it will just drop RPMs. Shutting off PTO has no effect, lowering throttle has no effect. Just before the RPM's come back there is a low pop noise seemingly coming from muffler. Sometimes the RPM drop happens quickly, other times after hours of mowing. When RPMs come back it may run fine for hours or days, or go out again in minutes. Any ideas?