Just to clarify, you took your LG to the same bedroom as the Dynex and connected it exactly the same from the same Mediaroom box with the very same component cables? With a direct swapout comparison like that you could easily start a warranty claim if only the Dynex has the dropouts. If both TVs do it from that specific location, using the same Mediaroom box and cables, then the problem is not with your TVs but likely with the Telus Mediaroom box or the Mediaroom service itself.
Best Buy's phone staffer just made that up. If it was widespread there would be a huge revolt from TV buyers, wouldn't there? Ignore that nonsense.it's a well known, very common occurrence with newer tv's as they are more sensitive