I had a similar problem in the past before with my Kinect XBox 360. The problem might be your HDMI cables. There are 4 kinds of HDMI cables according to the amount of bandwidth it could carry. I did try with my existing computer HDMI cable, it works but occassionally it doesn't show up especially when you set it to "P" rather than "I" . And you might have guess it right the P display a better resolution compared to the I setting. I did try to use spare HDMI which is a similar kind of low spec HDMI cables, problem still persist.
Then, I decided to buy this 2 metre monster awesome cable, which cost me $280++ but worth it, it can support above 10 gigabit of bandwidth which means support higher 1080P and above (for future requirement). Give it a try, you won't be sorry.
Thank you for the suggestion. But the thing is that, as I stated, the tv does output 1080p no problem. It is when the tv tries to change resolution to
720p things start to go awry. So bandwidth cannot be the issue here. Anyway, I set up an RMA, my set will be picked up for service early next week.
Hopefully things will go better this time.