Briggs 22hp VTek problems

I have a john deere zero turn that I can’t figure out the problem.
initially the problem was after about 20 minutes of mowing the mower would lose power sputter and eventually die. Sometimess run for another five minutes.
I have replaced the fuel pump, carburetor, fuel filter and vent tube from the valve cover. All new. Before this I replaced air filter and spark plugs.
It seems to be running better except surging. Idle up to near full speed.
What am I missing?
Thanks for any help.

Try running with the gas cap loose.
 
It may not be venting which would stop fuel to flow. 
And the fuller the tank, the worse the situation would be..
 
.

SRTsFZ6
Try running with the gas cap loose.
 
It may not be venting which would stop fuel to flow. 
And the fuller the tank, the worse the situation would be..
 
.

thanks but I tried that. I also blew out the vent line with compressed air fron the carb intake body with the cap off. I saw vapors rushing out of the tank.

Check for spark (plug out, hooked to wire, grounded to head) when the issue 
occurs.  
 
You could have a coil starting to fail..
 
.