Your cable run is not long enough to affect the TV's signal strength, but 3 TVs splitting the signal power could be putting your signal level on the Hitachi right at the threshold of being a "weak" signal. Also, any splitter that splits the Main Aerial signal could have a bad connection where the Hitachi gets less signal than the LG or Wharfdale does.
What I would want to try next is this --
Can you remove ONE TV connection at your splitter if it's easy to access, and MOVE the Hitachi cable connection to that connection ? First, if you remove ONE connection, it sends MORE POWER to the Hitachi, and second, if you move the Hitachi cable to either the LG or Wharfdale connection on the splitter, then you're connecting it to one you KNOW has never caused you any problems.
I've seen splitter connectors go bad, and I've seen too many TVs on one aerial.
We need to verify if the Hitachi is bad, or if we have too many TVs or if the Hitachi splitter connector is creating an excessively low signal to it.
And with Digital TV signals, you can't see the picture change from good to Noisy like we could with the older analog signals. I have to suspect less signal is present than what we need to provide rock solid operation of your Hitachi.
And I suspect it because of what you stated --
its just annoying as it seems to be okay for quite a while then comes on like this.
That's usually a weak signal just on that threshold of good and bad signal.