When my N97 is not open (not showing keyboard) after a short while the screen key lock becomes active, and then the my location on the map seems to fall behind. I'm driving along, at a constant 70mph (speed limit), and my device is showing my speed at 40mph, and 200mph, and flicking between them both. Each time it goes to 200mph I get the speed limit warning. The device was also lagging showing me about half a mile behind where I actually was.
On the return journy I opened my N97 (showing the keyboard) and the screen never locked, and my speed was constant (although 70mph shows 66mph on the sat nav but think thats down to tyre ware and things as it does it on all sat navs). I didn't lag at all with my location.