I assume it will be a hardware limitation. It seems to me that Humax specify the machine to be able to do the the maximum load the standard firmware will allow eg simultaneously recording two HD streams and playing back another HD stream plus maybe a bit of allowance for DLNA playback. Of course the custom firmware can impose a much higher load. The first candidate to investigate would I guess be the CPU.
shall i take it that all custom firmware users get the fairly frequent breakup of image/sound?