K-r & K-3 Bulb mode

Hi - I currently have the K-r AND k-3 models. I had reason to shoot with my K-r in bulb mode for the first time the other day but every time I tries to shoot for longer than 30 seconds the camera just seemed to switch off - tried new batteries just in case but with the same result!
Have recently treated myself to the K-3 but this would appear to have the same problem in bulb mode.
Am I missing something?
Is anyone else having this problem?

Thanks John - I'll check it out