transmission problems


the automatic gearbox in my charade has started playing games
recently..i've had a few scares when i go to overtake,the gearbox
drops down a gear as per normal but its like its not engaging correctly-
the engine revs up but the car goes nowhere forcing me to pull in again
at which point the gear suddenly engages and the car jumps forward!! its
unnerving to say the least!! anyone seen this happen before??