Do you have an aftermarket tune? I used to have the same problem and my tuner had to adjust the tune to keep the RPM's up. It started happening shortly after I modded the car.

My mechanic doesn't coast and he keeps the clutch pushed in at stoplights. I, however, will coast and pop it into neutral every once in awhile to slow down. Hence why he tuned it that way.

If it hasn't been tuned aftermarket....check your TB like other people said.