Car sputters when shifting from reverse to drive?

I drive a 2000 Nissan Sentra and for the past few months my car has been randomly sputtering but only when i'm driving over 50 mph but now it seems to hesitate to accelerate and sputters when I switch from reverse to drive. If I take my foot off the gas for a few seconds it's ok. What could be the problem? What would be the least expensive solution?