John Deere 170 Keeps Dying?

Last week I cut the grass and about 1/2 way thru I had to put in gas so I filled it up. About 20 minutes later it started acting like it was going to die. It was sputtering and didn't sound the same. Today I tried using it and it ran strong for about 5 minutes into cutting then started again sputtering and dying. The strange thing is it's worse when I go up a hill and if I turn off the blades it doesn't die. Any suggestions?