Different digital boxes and tv's have different levels of RF gain. Some need more signal and some need less to work efficiently.
Possibly your Bush pvr needs a much stronger signal than your tv so the available signal is permanently on the threshold of reception and atmospheric conditions will some of the time reduce the available signal when reception will become impossible for the box but still be above the threshold for the tv set.

Modern Bush is not reknown for being a high quality brand and the purchasers of budget brand pvr's often discover reception or performance problems in fringe signal areas. Because a single aerial must receive a very wide range of frequencies it will be more efficient for some channels than others and so one channel could be a fringe channel and another channel produce a very strong frequency.

The signal strength and quality can be checked for each channel by selecting options in the menu in order to check this theory.

The problem might be solved by replacing the Bush pvr with a premium brand or replacing your aerial for a more efficient type. One of my friends had five different boxes before he gained a satisfactory performance and another friend three boxes and still has problems.
My installation has a new high gain wideband aerial on top of my 3 storey house and a distribution amplifier in the loft to keep the input lead short and the whole lot connected by very good quality double-wrapped co-ax.
I had to add different amounts of attenuation for each box or tv set but in spite of the signal being on average too strong and needing attenuating there are still channels for which the aerial is not as efficient and providing only a weak signal.

Try changing the tuner frequency, or rescan and select another channel 7 with different frequency.
CHannel 7 digital frequencies for Sydney are; 177.5 MHz, wollongong 599.5 MHz.