Hello, and sorry you're having problems. We are now connected on this website- which saves you money over having a Tech come to your home, and in many cases we can fix the problem you're having. In other cases, these things do require additional parts to fix. But I can help you with that as well.
The format of the TV is actually determined not by the TV, but by the source you're viewing on it. So, your Cable/SAT box is changing the format and not holding the 1080P signal- which can simply mean the programming you're watching is Standard Definition (SD), or that the box itself is failing and needs to be replaced.
To check this, actually view the display of the Cable/SAT box and see what resolution it's showing. This is the resolution it is sending to the TV- the TV has no control over this.
If you need more info, then please reply back with the name of your Cable Provider, and also the Make and Model off of the front of the box.
Feel free to reply back if you need more help, or want to reply back with more details. It does not cost you anything more to reply back once you've rated. Please do rate me positively (at least 3 STARS) and I will continue with you until this is sorted. The positive rating ensures I am credited for my work.