I'd be tempted to play with the standby timer, and just see what difference it makes - it might alter the problem, it might not - but this will just be a case of ruling out anything that "could" be upsetting the box.
Same with the power consumption - what's yours set to? (Eco, fast or active)
It certainly shouldn't take 45minutes to restart by any stretch - something's not right there. You can try going through the factory reset route (this doesn't wipe recordings), but I suspect that if the attempts mentioned above don't make any difference, then it's almost certainly got a fault.
You can then report it by either of the usual methods. Call in (150 from VM, 0345 4541111 otherwise) or wait on here for a staff response - that may take a day or so.
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