My thinking is that if the signal is weak / quality poor, the tuners might take longer to "lock on" to the new multiplex or service stream within. We need the signal and quality for all your multiplexes (quality should not be less than 100, strength should not be as much as 100).
The TV is showing the signal source because it thinks it has lost the source.
I don't know then. The only other thing I can think of is that your hard drive could use a clean-up. This will require you to either use the Humax reformat option (ditching your existing recordings) or to install the custom firmware and run the disk check from Telnet (this is a means to link a PC to the Humax and access a maintenance command console).
Is the Hummy connected to the TV via HDMI, SCART or both? You can rule out one or the other simply by removing one of the connections at a time and re testing.
Does it happen each time you change channels on the Hummy?