Make sure you have the latest PACE drivers. These can be downloaded from: www.paceap.com
Also, make sure that you have compatible version of Pro Tools and Windows. See Digidesign's compatibility page for more information (digidesign.com > Support > Compatibility).

One last thing you can do is optimize your PC. There could be a software/hardware conflict somewhere in your system. See Knowledgebase articles 30058 and 30083 for a step-by-step procedure.