Wow, from spotty tv reception to rip-off, gold plated HDMI cables (probably the "M" brand, and I don't mean Monoprice) this took an interesting turn. Yes, there is no real thing as an HDTV antenna. I have a 30 year old Winegard on my roof that I put up for better analog reception back in the day, that works perfectly to this day for HDTV. It's a UHF-VHF 79 active element antenna about 30' from ground. A big sucker. I'd get myself off of the couch and at the very least check connectors etc. Rule that out. Forget the second amp but make sure the first one is working properly. I'd even try it without the amp depending on how long your RG6 cable run is. You did use RG6 cable, right? However, my guess is that being as your antenna is in the attic, any slight interference (swaying tree branches, electrical interference in the attic, etc) is going to affect your reception. Digital reception is fairly easy in that you either get it, or you don't. The fact that it cuts in and out means that your signal is just not consistently reaching the antenna for lots of reasons. It could even be the ATSC tuner in your tv. Not all ATSC tuners are made the same, especially on older tvs.
The suggestion about posting your TVFool info is your first step. There are quite a few folks here who can decipher that for you and even suggest an alternate antenna. It's not impossible to get good HDTV reception with an indoor antenna (house or attic) but it can be a bit more challenging than mounting it on your rooftop.