Why would my Audi A4 lose power sometimes when its been raining?

Answer

Take it to a qualified mechanic for a diagnostic

Salt Corrosion on the terminals (only if you are near the sea) (unlikely in any case though)

Water can sometimes cause the spark to short to ground rather than making it to the spark plugs. This often happens if the spark cables are old, dirty, or damaged. You can check this with a flashlight and a spray bottle of water. Park the car in a dark location and open the hood. With the engine running, spray water around the distributor (if it has one), spark cables and spark plugs. You might see sparks or arcing in the dark. The engine may start to run roughly when you spray water on the problem location.