2000 Honda Civic accelerating strange

My 2000 Honda Civic, 4 cyl, 96000 miles was just in the shop for replacing the timing belt, water pump, O2 sensor, cv boot. Everything was repaired including replacing a trans axle. Have had it out of the shop about 5 days. Upon accelerating from a stop, the car seems to go into neutral. After a few seconds, it goes into its next gear (it's an automatic) and drives fine. Took it back to the mechanic and the car would not re-create the problem. Should I be concerned?