Everytime that I take a shower and I want the water to be warmer, I will turn back on the cold water, but everytime that I would do that a brief period of cold water will run through then the water will water back up again.
That is an unusual occourance. I usually put my hand on the shower head to keep from freezing or scalding the more tender parts.
Both hot and cold should be connected to a 1/2" line and thus have equal pressure on them. It may have something to do with the distance from the water heater or you have a mixing valve that works a little different.