<VV> Mower problem - NO Corvair

[Grant Young]

This is one of those shots in the dark, but I have struck out in attempts to solve a problem with my Toro Zero-Turn mower with a Kohler 27HP V2 engine, so I am hoping someone out there might be able to help diagnose. There are some Toro help sites, but no help there either. The engine stars and runs fine for the first 5-10 minutes, then suddenly goes into some sort of loss of power mode and backfires through the exhaust (enough to blow a hole in the muffler). Taking the load (blade drive) off helps, but still not much power, similar to having water or contamination in the gas. Then for no reason, after 5 minutes of this, it will jump back into the power mode and be fine for the next 15-20 minutes. Then another bout of power loss and back firing for another five minutes, (I just sit and wait, knowing it will recover) then I can finish the next 2 hours of mowing without incident. 

I have rebuilt the carburetor, changed the plugs and fuel pump, replaced all the fuel lines, fuel and air filters, and drained the tanks completely. Fuel octane doesn't make any difference nor did a bottle of Heet. I also use Sta-Bil ethanol treatment, but tried it once without it.  I could almost live with this except for the cost of patching the muffler each time I mow. Don't want to put on a new one until I find the fix. It is a fairly low hour 60" mower. 

A private reply is fine. Thanks much,
Grant