The tuners in different receiving devices (whether they be TVs, PVRs, DVD recorders or whatever) have differing 'sensitivities' [i.e. their ability to handle weak signals]. Your Humax device obviously has a higher sensitivity than the TV set with which you're experiencing problems.
You've described the classic symptoms of insufficient signal strength. If the signal from your aerial is divided by a simple (unamplified) splitter like one of these
http://www.maplin.co.uk/tv-and-satellite/aerials/aerial-splitters
then you should replace it with one of these
http://www.maplin.co.uk/2-way-aerial-booster-223503
If you've already got a signal amplifier in place you should get an aerial installer to replace both your aerial and the associated cabling.
I've posted about the reasons why signal strengths vary, here:
http://www.theanswerbank.co.uk/Technology/Question1198775.html
Chris