For the other participants in this forum, What is the year of manufacture or the age of the vehicle?
Could the issue maybe be attributed to a malfunctioning sensor? One possible explanation for the issue at hand might be a malfunctioning pedal sensor, as well as potential faults in the PCM or ECM components. This is only a conjecture.
Does it produce any error codes or messages? Was the mechanic able to inspect the diagnostic codes?
It is advisable to consider seeking assistance from an other technician, since it seems that the previous mechanic may have shown a reluctance to address the issue. Particularly if he abstained from doing a test drive, it might be inferred that he lacked interest in engaging with your business.