I'm not sure how dim 'dim' is, (ie the 'suspend' version of dim or the inactivity version) but it almost sounds to me like the 770's forgetting it's received input and puts you into the power saving mode where the screen dims after 5 mins.

Have you reflashed?

(I've also experienced the ultra dim 'Nokia' logo once before... popping the battery and rebooting solved the issue)