[EqUaTe]

To be honest that sounds like a faulty STB... Either that or faulty cabling.

Try running your main telly off the analogue tuning (with or w/o a vcr or some other device in between), and see what happens.. Just a thought.
If it does the same, call them up and complain.. Explain the problem, tell them you want to complain.. You'll likely get a much quicker response that way...