My dishwasher (I'm sure this is making a repeat mention in these pages).
It won't clean the dishes properly, despite my trying two tablets (in the rack as well as the dispenser) ... half the time it won't even release the one in the dispenser.
I have had the repairman around twice, and he cannot find anything wrong with it. The first time he came, it performed perfectly on a 90 minute cycle (he went to another job and returned later), and continued working well for a month. Then it started playing up again, so I called him out. Again, it worked perfectly for him - just doing exactly what I do - and continued working well for a couple of weeks.
Now, it's playing up again and it's driving me maaaaaad! I even tried different tablets, and powder, in it, to no avail. Plus I bought a cleaning thing and ran it through, and have checked its waste bits and pieces aren't clogged. I'm about to throw the darned thing out the window! The repairman is booked for Thursday, which feels far too far away.