The CR18 is a pretty stout rim; I've been pounding on a pair of them for about 20,000 miles with my weight ranging from 265 to my current 210-ish, and I carry commuting gear upwards of 25 pounds depending on the day.
Unless you really pranged that rim and put a dent or blip in the bead, I can't imagine that it is far enough out of true that it is unfixable. I also ride Pasela TG 32mm tires, and they offer a fair amount of cushioning against rim damage. If you didn't end up with a pinch flat, you probably didn't clang up the rim to an unusable state.
Sounds like Bill is correct in stating that your mechanic is just turning spokes until the wheel is straight, without regards to proper balanced tension, and a slight jarring is enough to pull the whole thing back out of whack. The wheel needs to be brought down to zero tension and trued/tensioned from scratch.
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