Same problem here. I purchased Navigon without Traffic and had no problems at all. After I installed the Traffic feature-downloaded directly through the app- It now asks me for the iTunes password and as I'm driving the speed jumps from actual speed to 0 and back to actual speed very rapidly. This causes the map to stop moving and the move again creating an unusable map. Not happy! I'll try uninstalling as suggested by others here and post results. Installed on iPad2. Haven't tried on iPhone yet.
Okay, today 4-29-11, I reinstalled app on Ipad2 and Itunes, then reinstalled by downloading from Itunes and syncing. This did not solve the itunes password request, but happily, it did fix the speed/map jumping issue. I can handle putting in the password until Navigon fixes the password bug. Although I haven't had a chance to really put the app through it's paces, I am quite happy with the app. Much better than my previous 2 Garmin Nuvis.