18 HP B & S engine stalls uphill?

I have a craftsman lawn tractor with a B & S 18 HP Vtech engine. It is three years old and did not have any problem until this year. I am mowing the same yard and have a small incline in the front yard. I have always mowed it vertically but now after 2-3 trips up the hill, the tractor sputters, backfires and quits and will not start for 15 minutes and then it is fine as long as I do not go up a hill (not talking a steep hill at all). Any suggestions are appreciated
Gary