air conditioner in my 1999 Tahoe

adam r, part of this is when you are moving, the forced air passing through your a/c condensor is dissipating the heat. When your stopped or moving slow, not enough air is moving through the condensor to move that heat out. If your a/c does not cool down properly when not moving could be because of the fan on the condenser (or a problem in controlling the fan). The other possibility is that when the engine is idling that it is not turning the compressor fast enough to work efficiently. This could be a sign that the freon level is low.