AC Cold at road speed not at idle
 

I have a 2000 Blazer with 181000 miles. The AC has been blowing cold all summer up to a couple of weeks ago. Now it is cold as long as the engine is running at moderate to high rpm but does not cool at all when idling or lower rpms. Is this an indication of low freon? Also I have noticed a ticking sound coming from the AC compressor lately. Could this be related?

Without knowing what the high and low side pressures are when the clutch cycles, it could be low on refrigerant, could be a problem with the orifice tube, the fan clutch may be faulty, or the condenser is restricted either internally or externally, (leaves, debris etc).