Four basic possibilities:
4) Random insanity
We can try to rule out the first 3. Can you post your line stats? Odds are this has nothing to do with it since you get good daytime speeds but might as well check. As for POI and node...if it's POI, every Teksavvy customer on that POI will be having similar issues. I don't think that's the case, but someone from Tek can check into it. For Node, the only way to tell is to ask neighbors on cable internet to check their speeds at peak hours...if it's node, odds are at least some of them will be experiencing similar issues.