I have a 7 year old Sears 7Hp self propelled 21" mower; model # 917.377820. I bought it to specifically cut a 200' by 15' hillside. Initially it worked very well but as of late it has begun to sputter and die after about 15 to 20 minutes. This would be the third pass along the 200' length. The hillside is inclined at around 25 to 35 degrees. Once it dies there is the strong smell of gasoline and it will not restart unless it sat on level for about 15 minutes. The oil level is fine and the gas tank always near full. I've also changed the airfilter as this mower seems to be very sensitive to even modestly dirty airfilters.

Any suggestions.