If you double click the character portraits, hit F1 or F2 twice (IIRC the default hotkeys for character selection) or left click on the ground and hold (to have the characters move in the direction of the cursor), does that get the camera to lock back on to the lead character?

Have you tried changing the game resolution, to see if that would make a difference?

You could try windowed mode: [Info] Debug Mode - Better FPS + Windowed Mode