Projector shuts down after only a few minutes.

After somewhere between 10-30 mins of operation, the projector seems to turn itself off.

The projector will blank out and the red 'lamp' light blinks. The fan continues for about 30 seconds (which seems shorter than normal?) and when complete the orange 'Power' light goes on, indicating 'Stand-by' mode.

At this point, I can turn the projector on again and get another 10 minutes.

I have tried to clean out the unit - did I miss some important, easy to miss, vent?
I have just replaced the lamp, and though the image is brighter, this shut-off behaviour still occurs.

Is this temperature related?
Is this a problem with the projector itself?
Is my second lamp defective?

Thanks for any help you can offer.