Murray rider with a 17.5 briggs losing power

Just recently my mower has decided it wants to lose power. At first I thought it was because the grass was kind of long and I was going up hill.
Then today while mowing it started to sputter and would continue even when I disengaged the blade and was stopped on flat land. It did continue to run when I put the throttle down to idle but when trying to put it back to full speed it would start cutting out again. I shut it off and blew out the art filter and I thought that helped, but then it started doing it again. Then I shut it down and let it sit for a couple of hours and it fired back up and ran fine for about 20 mins. Then it did it again and this time died even at low idle. I let it sit for another couple of hours and it started right back up????
Please help and give me some idea's. I really need this mower to last for a couple of more seasons without any costly repairs as I really don't have a new mower in the budget right now.
Lastly I have had the mower for about 13 years (got it new) and I will admit I have not kept up on maintenance like I should.

Several causes
Most common are
1) magneto over heating & breaking down
2) debris in fuel tank blocking off fuel supply
3) Blocked fuel filter slowing down fuel supply
4) ditto for the carb
5) overheating engine causing the valves to stick
6) oil level too low