That's bizarre. You either have a wild coincidence at play, or something in your environment is causing this to happen. You have to look for commonality.
You already eliminated an issue with the antenna wiring by moving the unit to other rooms.
Do you have the unit connected to the phone line? I'm guessing here, but one thing that can play havoc with microelectronics is a grounding problem. When a device connects to both the electrical line and the phone line (or cable TV), a floating ground can occur. In the US, many building codes now require the phone and electrical ground to be the same (e.g., tied together). It used to be OK to ground the phone line to a copper water pipe. The electrical panel is always grounded to a copper grounding rod. Even though both are "in the ground", a voltage potential can exist between them. If an electronic device is not specifically designed to defend against this, a floating ground can damage or create bias on components. A bias issue may clear up if the grounding issue is resolved, so if your unit is plugged into the phone line, try unplugging it.
I used to live in Mayaguez and I lost three of my four GE PBX-type phones over a period of several months (plus I lost one of the replacements). I finally realized what was going on. My phone line was grounded to a copper pipe and I had a floating ground. Once I added a combo surge protector to each phone, no more bad phones. Interestingly, a GE engineer admitted they had a lot of problems with this, and that the phones were not adequately protected internally to deal with this.
I recall my first DISHPlayer came with a combo surge protector in the box. My 508 did not, but I always use one.
Another possibility. I assume you got a new smart card with your replacement 508? Otherwise, the card is a common element, although I don't see how a problem with the card could cause your symptoms.
As for the 2-day guide issue, I think it's unrelated to the "signal" problem. If there was a real signal problem with the download, your two day guide would not update. I recall there being a key code to force a full 9-day EPG download. Perhaps someone else here knows what it is?