It's a straight forward effort to check the der's movement freedom, then the cable's, then the lever's. It's rare that a movement problem comes and goes. About the only reasons that I can think of are road crap getting wedged in the der parallelogram (and then drop out only to have another get jammed in, then again fall out.....). A cable that is starting to fray at a rub/port point. But this usually gets worse and worse, again not likely to fix then "break" then fix over and over. Then there's cable corrosion. But like a fraying cable rust never sleeps, then awakes, then sleeps, then awakes. It is possible that the shifter's internals are the problem. But ONCE AGAIN, these types of mechanical issues usually only get worse and more frequent and noy fix themselves over and over. There are some situations where a too tight cable WRT the limit screw adjustments can cause a lever to not release. But this is well known by good mechanics so if this is the problem and your shop didn't catch this (or created this when they last worked on the bike) you might seek out a different wrench.

So we're left with the wild card which can be very different form mile to mile. But before we go there have a competent wrench check every thing out fully. Consider having the cable replaced as much as a preventative step as one that might reveal something.

After all the mechanical aspects are found to be in good order take a look in the mirror. The rider is the wild card.

I have gone out on a bit of a limb without having any real info. Just a lot of experience in seeing fellow riders not shifting well at times. I hope I'm off base since fixing a mechanical issue is just $ and not changing behavior. Andy.