We fully understand your concerns with a technician call out charge, we would only charge you if it was your equipment at fault, but as you have advised you have already tried two new leads and the problem still persist it may be an issues with our equipment.
Have you tried another TV? You can try our cable box with another TV to see if you are experiencing the same issues. Also Is the problem on every channel or just with our on demand services? Sometimes a reboot of the box can make a difference have you tried this at all?
Can you check the above and let us know how you get on?