I own a 5080 also and I use DirecTV so the names of the settings I'm going to give you might be different from what your FIOS system uses, but I think you'll be able to figure it out.
With DirecTV there are two different settings for the resolution output of the box. The first setting is "Native", which means that the box outputs the same resolution as the channel that you're watching, 480i, 720p, or 1080i. When the box does this your TV has to do the switching between the different resolutions every time you change the channel. That is the distortion that you are seeing when you change the channel between HD and SD channels. If you notice in the upper right hand corner you will see the different resolutions displayed.
The advantage to using this setting is that I have found that the 5080 does a better job of displaying the SD channels at this setting than the DirecTV box.
The disadvantage is what you wrote about, which is when you change channels between HD and SD you get that distortion for a few seconds while the TV is reformatting itself. It will also do this if the HD channels are say 720p switching to 1080i. Not all HD channels are 1080i. Some are broadcast in 720p, such as FOX network, and other "cable" HD Channels.
The other setting on the DirecTV box is to turn the Native setting off. What that does is it makes the box output everything in 1080i resolution and at whatever picture setting that you set. This is the setting that the DirecTV installers always set the box to. At this setting your TV doesn't have to do any internal switching so you don't get the distortion when you are changing channels.
What you might want to do to use this setting is go to your setup menu on your box, then go to the HD settings, and see what your setting is set at. It probably will say "Native" or something like that. There should be a way to turn that off. That should take care of your problem.
I hope this helped, please let me know if this worked.
Has anyone experienced any strange behavior on your sets when switching between HD channels and SD (and vice versa). I just noticed that when I switch channels there is bad image distortion...horizontal line across the screen and a broken picture (or several) displaced into the corners of the screen.
I'm using hdmi attached to a fios box...