Using TV as a Monitor with HDMI - windows no longer centered when I switch over!

Hello, I ran into a very annoying problem this weekend. For a long time, I've just been using a TV as my computer monitor when I want to watch Hulu, Netflix, etc. I just unplug the monitor from my Mac Mini and plug in the HDMI cord I have hooked up to the TV. It's worked very well for a long time (years) but this weekend when I unpluged the regular monitor and plug in the TV HDMI cord, the windows are no longer centered on the TV! The movie or browser window would be in one of the corners and when I unplugged the TV and replugged in the regular monitor, the window would be in a completely different spot! I'm not sure why it started to do this or how I can get it to behave properly again. I've tried playing with the overscan settings but nothing. 😟 Any ideas?