One of my HDRs has started doing the same thing recently. I haven't actually timed it but it's fine for some minutes before it decides to power down the aerial pass through. It has never done this before.
There is an intrinsic Humax design fault in the pass thru operation which has bugged me for sometime whereby some boxes always power down the aerial pass through even when power saving is not selected I've only seen this on earlier models where the aerial in/out sockets are side by side rather than later models with the in/out sockets one above the other. I had always assumed that it was a batch related hardware problem but this latest observation may indicate a software bug. The only workaround I know of is to use a splitter and avoid using the pass through facility altogether as Black Hole suggests.
If you do attempt a reset I'd be interested to know if it worked.