Stihl electric strimmer cutting out

I bought a Stihl electric strimmer about two years ago and, on using it to strim the grass between my veg beds, found that sometimes when I stopped it to move something or whatever, it wouldn't start again, so I put it back in the shed and tried the next day and it was ok. After this had happened a few times, I took it back to the shop who were very good and, though they couldn't find the fault (perhaps because they weren't strimming with it and not running and stopping it long enough?), they replaced the starting switch. It is still doing it, though I haven't been back to them as I don't know what else they could do. The motor casing doesn't feel warm and it doesn't do it every time - very unpredictable and frustrating. Any ideas?

slight break in the cable that disconnects when it is bent 'just so' either just where it leaves the plug or where it enters the machine (we have a hoover that does this but only cuts out when the wire is bent in a certain direction at just the wrong point).
Grendel

Along with Grendel, I wondered about a damaged wire in a mains type machine, but thought that any workshop worth it's name would check for that.
I expect that their maybe some manner of thermal cutout on the motor that is kicking in when you stop the motor. Although the casing may not feel warm, it could be that the motor is, but that would depend on things like how long you are using it for, how long the "string" is, whether or not the automatic string cutter has been removed or has fallen off, or that the machine is just plain faulty and should have been fixed under warranty.
I would get back in touch with the company that looked at it last time and tell them that it is still the same. I would also get in touch with Stilh UK customer services and tell them that you ain't happy.

I have had this same problem on petrol machines that have small tolerances on the dead-mans-handle mechanism. When the engine got hot and everything expanded, the cut out switches would engage stopping the engine. It was a simple fix, but was very annoying for the operator until it was. I wonder if something similar is happening with the switch on your machine. The fact that the machine takes a couple of hours to get so you can start it again would suggest that it is (to me, anyway)
Phone Stihl, they are very keen that you remain a happy punter.