Your switch assembly might be pooched as ThunderCactus said, It seems that your trigger assembly is completely covered in carbon from arc's.. So when you pull the trigger fast, it scrapes some of the carbon off exposing copper which makes a full connection and causes it to fire, while when you pull slow.. it barely/doesnt remove the oxidation layer so it only fires when it completes a circuit, which is whenever the trigger assembly finally dislodges from the contacts and makes a complete circuit... thats my 2 cents of whats wrong haha
