while on a hill and in drive

[moxwell]

I was in drive at a standstill while on a hill and the car started to roll backwards(automatic trans). It was a little unnerving as i needed to maintain steady accelerator pressure to keep the car from rolling back. Is this a transmission or engine problem and what maintenance can be done to cure this. on another topic: I was considering re-sleeving the cylinders, is this an expensive procedure?