This morning we released an update to our JavaScript API. Version 5.2.1 uses DOJO 1.0.2, which fixes a bug in the original DOJO 0.9.0 GFX library that could cause line overlays to "jump" at certain zoom levels. If you do not use overlays in the 5.2 version of the JavaScript API, you do not need to make any changes.

To update, simply change the version parameter to "v=5.2.1":
<script src="http://btilelog.access.mapquest.com/tilelog/ transaction?
transaction=script&key=YOUR_KEY_HERE&ipr=true&itk=true& v=5.2.1" type="text/javascript"></script>