Random issues with C6300

Hi all,

I've been looking through the forums for answers to these questions, but nothing seems really applicable to these problems, so I guess I'll start with a bit of backstory:

Recently our internet service was boosted to new, higher speeds from our ISP.  The end result was that our old modem no longer suited our needs if we wanted access to the new speeds.  We decided to upgrade to get access to those speeds, and so purchased the C6300 to ensure compatibility (checked with the ISP first, it is a suitable router for the plan we are on).  The installation went fine, except for my ISP having to send a tech out to finish the install, since the modem was not showing up as active on their end [turned out that, after repeating them 3 times, they had incorrectly entered the MAC and Serial Numbers], but while he was here he checked everything.  There are no splitters in the house, we have a solid connection running to the house and everything was in perfect order by the time he left (he also came back about five minutes later to make sure everything was still in top shape, since the ISP had contacted him to say they noticed a small hiccup on our end, everything was still fine).  Then, a couple of hours later, we started having a couple of issues:

-Speeds on my hardwired PC are fantastic, they actually sit a bit above the new max speeds we were told (http://www.speedtest.net/my-result/4695796276), but when connecting through WiFi everything feels sluggish, and speeds rarely top 25Mbps down / 2Mbps up.  I've checked the settings over and over, but everything seems to be set correctly.  The C6300 is in the exact same place as the previous modem/router used to be, and we had zero issues there.  Am I overlooking something simple?  For instance, would changing to a different channel help? 

-We also seem to be suffering from random disconnects, but they aren't really disconnects, more like momentary lapses.  Randomly while using computers or laptops on WiFi or, more often, towards the later hours at night when using an Xbox One we'll get random "attempting to connect to server" messages for any game that attempts to connect to any online service, or we'll randomly get bumped to "Limited" internet access on computers.  The hardwired PC remains perfectly fine, but everything else on WiFi seems to lapse, and it's only a momentary thing, not long-term, but it will happen more than once. 

-I'm also seeing this message popping up a lot when I check the logs:

[DoS attack] AIF: Dropped INPUT packet: PROTO:2 SPT: DPT: 1 Saturday, 26 Sep 2015 07:22:41 224.0.0.1 10.128.64.1

I know it's not a legitimate DoS attack, but it seems strange that it continues to pop up like this routinely in the logs.  Is this something I need to be concerned about?

Any help in sorting this out would be greatly appreciated! :)