I have limited experience with cars. I bought a 2001 Mazda 626 to commute to and from school a few weeks ago. Today when merging on the freeway I noticed that the engine sounded like it was working too hard and was turning at between 4,000 and 4,500 RPMs at 60-65 MPH. At one point the car let out a single violent jerk (no change in driving/RPMs before and after). Off the freeway acceleration was poor and rough. I parked it, went inside, had a panic attack, and then took it for a second test drive. It was fine after having sat for about 10-15 minutes.
The previous owner reported experiencing this problem 1-2 times. He had taken it to a mechanic prior to selling it to get a recommendation on the transmission, but the problem could not be reproduced in test drives. Fluid levels were good. The seller gave me the maintenance records, should I call the same mechanic or seek a different one? Should I request that the mechanic do more than test drive the vehicle to try to diagnose a possible transmission problem that is presently inconsistent?