If the chainguide is setup correctly, there shouldn't be an issue. I would make sure that you are running the guide wheel the right way. There is a thinner and a thicker profile on them which can sometimes affect the chainline. Also, make sure that the guide wheel is where it should be as you can adjust where the wheel is.
If the chain is lifting off the chainring and then dropping back down again it may be a chain or ring issue or if it is only happening in the top one or two larger cogs it could even be a chain line problem. I have used MRP for nearly the last decade and can't remember ever loosing a chain. You should not have to cut, file, or change anything in order to keep the chain on though, provided that it is set up right. Chain guides are tricky sometimes....it would be worth going to the shop, or back to the shop, to sort it out. Don't modify anything yet.
If you always stay in 3 and 4 they may be just worn out, happens all the time. Even though it is uncommon I've seen chainrings worn bad enough that despite the amount of contact the chain will still slip under load on them. It sounds like you've worn out your 3rd and 4th gear and/or the chain and maybe the ring even. Still could be chainline though.
What it does not sound like is a chainguide problem though. Sorry!
Looking at your album, it doesn't really look like the top roller on the guide is actually guiding. Rotate it counter clockwise a little bit, and see if that does anything.
Well, hate being defeated but it looks like I am. The only other thing may be a stiff link in the chain. Does the skipping happen at the same point at each pedal stroke, or same time if you keep pedaling at the same cadence?? That is a sure sign of a chain issue although I would hope that that would have got sorted on one of your six visits to the shop.
It still does not sound like a chainguide problem to me though. If it has been happening since new for sure then take it back to the shop and politely tell them the issue and that its been there since new. Its up to them to sort it out....