When I'm on a community street with houses in Tallish Crest, for example, I used to think there was terrible lag and the system was infrequently detecting my key-presses, because I'd often get stuck running in one direction. I could sometimes interrupt this a bit, but mostly it would just run me off cliffs and into walls and generally make me swear.
I finally figured out that what was really happening is that it was interpreting the mouse pointer as the relation location to which I wanted to go, like a carrot dangling in front of a particularly stupid horse. I'm not sure what's different about the place. I thought it might have been because you have to click an address and that tends to leave the mouse pointer in the middle of the screen, but I select stuff on the screen all the time, and this behavior only seems to show up on local streets.
Wiggling the mouse around and/or moving the pointer outside the play area solves the problem.