Have a Z665 that runs for approx 2 - 3 minutes cutting grass and it starts to stall, shut down mower and some times it was run ( very rough) and other time it may shut down. Can restart right away, it idles rough for a few seconds and idle evens out. then this will keep repeating if i try to mow again. I have changed spark plugs, fuel filer and air cleaner. No change. Looking for suggestions for what it may be or how to solve the problem. Thanks