I've had my Saris Powertap setup for a long time and its been through hell with me. I rode it in nasty weather for multiple years (its the original, yellow cover, version). Now it's not working properly. It reads everything properly except for power. Sometimes the number is spot on (based on perceived effort) other times it appears to be either 100w low, and other time 100w high. Has anyone else encountered an issue like this? Any ideas on how to fix it? Other than buying a new one