I'd suggest replacing any cable Frontier runs with Cat5. I don't let them run their low quality cable in my house, so I installed it all myself. It's really quite easy, as normal phones only use one wire pair and Cat5 has 4 wire pairs. You may see better margins with higher quality cable in the house. Also, avoiding any splitters and couplers will help.
There should be a test jack on the NID. Plug the modem into it with a short known good cable and test the margins and speeds. If things improve, you've found your issue. However, I doubt this to be the problem, as you said it was okay at higher speeds before the problems started.
I wish I could tell you exactly what Frontier did to my friend's line to get it fixed, but the best solution is simply to track down the line from your house to the CO and redo every termination.--
CompTIA Net+ Network Administrator - I know networks!
Professional Photographer - www.jnphoto.biz - Weddings and Senior Photos
Nice and comfy with Frontier DSL: I can help with your issues!
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