High discard transmit count and TCP Out-of-order issue

I noticed in my monitoring software (Solarwinds NPM) the other day that the interface connecting our main switch stack and our router has over 20 million (million!) transmit discards and climbing. My network has about 150-200 devices on it, this discard amount seems ridiculously high to me. We aren't having any noticeable network slowness, and had I not looked at this particular screen in the monitor software I wouldn't have known there was an issue at all. That being said, it was time to investigate.

I ran a "sh int" on my switch stack interface (gig speed) on my stack of 4 Cisco 3750 switches. Nothing looked unusual. Ran the same command on my core router (Cisco 2821) inside interface (gig speed). One thing stood out: 1237956 unknown protocol drops and rising. It's not 20 million but it's a good start.

Tracked down this article: https://supportforums.cisco.com/docs/DOC-15490 and made sure DTP, CDP, LLDP, VTP are not applied/enable on either where applicable. Checked, protocol drop count was still going up.

SPAN’d (mirrored) the switch port we're having problems with to sniff the traffic with Wireshark. Results show I’m getting a lot of “TCP Retransmission” “TCP Out-of-order” and “TCP Dup ACK”. From my research these errors show a bottle neck somewhere, I'm unsure of where that bottleneck is as everything is gigabit speed leading from the PC out to the internet. PC <--gig--> Switch <--gig--> Router <--gig--> Firewall.

My question: where do I go from here as far as figuring out where this bottleneck is? Or alternatively if it's not a bottleneck, what settings need to be changed to fix the unknown protocol drops?