When I once had the issues you described it was the cable binding randomly at times. When I finally found the cause a couple weeks later, it turned out to be the cables cutting into the steel of my BB shell where they ran bare around it as many did in the vintage days. A cable guide solved it. Perhaps your cable guide on the BB is worn out.
For sure I'd go back and inquire with the mechanic as to what else they recommend. Besides you, they know the most about your specific bicycle and what was done to it.