There are three possible cases that could cause this problem. First, you just have a sticky trigger or something lodged into the trigger mechanism. This is a relatively easy fix and just requires cleaning the trigger. If necessary you can open up the casing and remove any debris surrounding the trigger. If that doesn't help it could be that your trigger mechanism is broken and needs replacing. If that doesn't help then it means that your wiring is bad. Instead of using friction brakes, drivers reverse their polarity in their engines in order to slow themselves down. There is probably something wrong with how the wire are connected and you should rewire your driver to match that of the wiring diagram of the parts list which can be found on the main device page. Your question can also be answered on the RIDGID X4 18V Lithium-Ion Impact Driver Troubleshooting page.