I agree with sewing at the shop until you notice the problem.
I had a weird problem with my machine - it would change stitches without warning, after I had sewed for awhile. I took it to be repaired, and they did the basic maintenance, and sent it home with me. The problem was not fixed. Took it back, and the repairman sat and sewed on it until it acted up. I was sure he would give up before there was a problem, but he stuck with it while we chatted, and it did act up.