Help - need a reliable repair option

Our robot went in for "repair" after it stopped working last summer. We just got it back after we were told nothing was wrong - it ran just fine for them. It ran for us less than 2 hours, then shut off (had picked up some debris but less than would expect). We cleaned it, returned it to the pool and it did the same. Normally this unit runs for 6 hours before shutting off. Ran it again without the bag (this is an older model) to simulate what the repair shop had done and it stopped working all together. Did we just burn out the motor? Is it worth the repair bill (the unit is at least 7 years old, but has always worked great for us)? Where can we send it since the bozos here we really don't trust! We would like to fix this guy, quite fond of it. Thanks for any advice, we are in Maine.