Problem with HD Picture via MONSTER 400 HDMI onto Pana 42dbCab

Hi all , just a question for the technically minded here.

Viewing SKY HD using the above I am getting a terrible picture far worse that an upscaled version of the same picture , think Sky one and Sky one HD , picture on the later is very grainy with artifacts in the dark areas.

Thing is that when the engineer installed it it was superb.

I changed the resolution to 726p , just testing the set up and then I put the box back to 1080i. That is when the problem seemed to appear.

So I have switched it off , not done a full reboot yet etc etc.

I have to ask if anyone else has had a worse picture using a similar setup after changing resolutions.

IF so have you corrected it and how did you achieve this.

I have never started a thread and am fairly new to this having replied once to someone else's post. So if this has already been posted some where else I beg your pardon in advance.

PS. what do any of you think about monster cables.

If I was you I would reset the box, if the problem still happens give Sky a call.