2001 Ford E-Series Van sputtering during acceleration

I have 2001 e150 cargo van ( 6 cyl. 83,000 mi.)that has jsut developed an intermittent sputter during acceleration. when traveling at 70 mph, I let off the gas when I reach a decline. At the bottom of the decline I accelerate and as I begin to go uphill, the engine sputters. If I continue presing the gas the sputter continues, if I let off the gas and accelerate again it will stop sometimes. This always occurs around 50mph but I have not noticed it from a dead stop. I do not notice a rough idle problem .
Any ideas as to what order I should try replacing parts?