... the DAT indicator only blinks on briefly occasionally. It never gets sold. Does that sound like a simulcast issue?
Yes - but it could mean other things as well.
If you are only scanning it as a single system/site (or holding on the site), you should see a fairly steady pattern of DAT - like, on for a second and then of and then right back on again. If not, the radio just isn't receiving the signal steadily enough to work well.
The first thing you need to accomplish is finding a way (location) to receive the system/site consistently. From my experience, the best/easiest way to do that is move/drive towards a transmitter site. The idea here is that if you're much closer to one transmitter tower, you are manually reducing the simulcast effects. Once you are able to do that and receive something then you can start considering options (antennas, etc.). I'm just not convinced yet that you have things configured properly yet - but it sounds like you're getting close.