I have a 721 grasshopper with a 52 inch deck. It runs great for the first 5-6 minutes of mowing then it sounds like it starts missing and looses power. It almost dies. I shut off deck and after a few second it winds back up but, not to full power as before. I can keep mowing to get the lawn mowed but, it just limps along the remaining time. I have ran it for 10 minutes with out deck going under no load and it runs fine. As soon as I put deck in motion 4-5 minutes later it will start missing even just sitting still under no load. I have replaced both air filters, fuel, oil, hyd filters. Also i am not sure why but, when I bought my mower it has a in line fuel filter on the return line back to the tank. I have replaced that also but, not sure why that is there? Any thoughts on why I am losing power after 5 minutes? things to look at? also do I need in line filter on return line?