String Trimmers

I am not an engineer, but will try to explain my problem with everyday terms. Last season, the shaft slipped out of the driver. If I hold the trimmer head end up in the air, the head turns, but when I hold it with the head down, as you would when using it to trim grass, the head no longer turns when I rev the engine. I took it to Eureka Rental who sold me a new shaft, both parts. The problem returned. My son and I figured out that the "C" clip does not hold the shaft in the driver so I bought a new clip. I used the trimmer for less than an hour and the shaft slipped again. The trimmer laid on a shelf for a few months. This season, my son had the idea to tack a "C" clip with his welder. The clip was too small to tack, so he tacked a small washer to the shaft where the clip usually sits. This repair allowed us to use the trimmer for about 2 hours, then today I used it for 10 minutes and it slipped again. We are  beyond frustration. Is there repair for this problem? My son says "just buy another one", but I thought I had bought a high quality brand and paid for it. My neighbor says he buys a cheap brand and  replaces it every year. He has three trimmers and at times none of them work. Please advise.