does high altitude make plugged egr passages issues worse?

What is your question?

drove the van (camper conversion) from Pennsylvania to Denver. It ran great the entire trip until we got off the interstate in Denver. Once driving on the streets it started running very rough and stalled at idle (at lights and stop signs). Would restart but still ran rough. Limped to our destination and after it sat an hour or so and cooled down, it started up and ran fine to the mechanic that we found that would see us. After they took it for a test drive, out on the Interstate and then back in town, they had the same issue. Decided the EGR passages (I think in the intake manifold) were clogged and they "cleaned" them and it ran fine afterwards. My question is: can I be fairly confident that the problem is really fixed? I would assume the passages were clogged before we got to Denver. But not badly enough to be a problem until we got a mile high?