car revs and struggles to accelerate

I changed the oil, brakes, and spark plugs on my friends car. After I was finished I cleared the "maint reqd" light off her dash , we put the car in reverse and the engine would build rpm fine but would barely move. Then I put in drive to pull away and the engine built rpms but struggled to gain speed. It gradually improved and does fine in a straight line but turning right it still revs significantly without out gaining speed then it "catches" and accelerates. Feels almost like riding a clutch then dumping it and will accelerate suddenly... Please help im stumped!