Vet inte om läst forumet men så här står står det i FAQ:
11. The GPS went crazy part-way through my run.
For best results, make sure wi-fi is off when you use RunKeeper, and use top lock to sleep the screen whenever possible. To address this specific issue, many users have found if you turn 'set clock automatically' off on the iPhone (iPhone→Settings→set clock automatically→off) it eliminates this issue (some carriers/regions do not have this setting). Also, make sure you have downloaded the latest version of RunKeeper.
12. My device can’t find a GPS signal at all, or the GPS signal never seems to be accurate.
This could be a number of things. First, make sure location services are turned on on your iPhone, and that the device is not in airplane mode.
Second, iPhone GPS can have issues if it is cloudy, or around thick trees or tall buildings. It works best outdoors with direct line of sight to the sky. This could also be a coverage issue, as certain areas to not have a strong signal.
Third, the issue could also be device-specific. Try powering off/on the device. Also, try a location warnings reset in iPhone settings, settings→general→reset→location warnings, as several users have said this helps.
Fourth, sometimes the GPS needs several minutes outside to recalibrate with the satellites in order to find a signal. Try setting the device down outside with RK open while you stretch and tie your shoes, and see if it is ready for you when you are ready to go!
Fifth, it is possible the app did not download to the phone correctly, try deleting/reinstalling the app to see if this helps.
If none of these work, I would take the device to Apple to have a look. You could also try one of the other free GPS fitness tracking applications, which will help isolate that the issue is RK-specific or that it is not RK-specific.