Just wanted to follow up as promised! The slipping problem was intermittant ... going away for weeks at a time. Lately it was occurring more frequently so took it to the transmission shop. They drove it but, of course, it didn't slip for them. They did get error codes for two transmission solenoids. The solenoids were replaced, car test driven and no more transmission error codes are showing up. Two weeks later and no more slipping (fingers crossed) and the acceleration is much quicker. It appears the malfunctioning solenoids were robbing power and making the car generally sluggish.
Cheers,
Austin