If a router doesn't know what to do with a packet, it drops it. packets [layer 3] are assigned a sequence number for reassembly at the receiving end so that after the packet and frame wrappers are stripped away and the payload is put on the physical medium, it's in the right order. If in reassembly of the packets in sequence at the receiving end results in a lost packet, the receiving side has to request that packet and the sending side has to resend it. This can cause a slow down. Lost packets do happen. Packets don't follow a single route because routing protocols such as OSPF find the shortest path, other routing protocols use metrics, but the gist is the sending side simply sends out the packets and hope the receiving end gets em all. If not the sending end relies upon the receiving end to ask for the missed packets. If there's a lot of packet or frame errors, this can cause network slow down.

But, slows downs can be almost anything on a WAN interface... from a single user sending and receiving large amounts of data, such as video, to latency at next hop router or even routers farther out in the inter-network. Inter-network routers don't alway use the same routing protocols between them. Some might use distance vector routing protocols, some not. I would on off hours do a debug on the WAN T1 interface, capture some of that information, analyze it to see what's going on. you could also use wireshark and capture packets, see what's being sent out and received and if anything funky is going on with the packets that cause excessive errors.

personally, I would look at what's being sent out and received on the WAN link. If you've got some users steaming videos, that's going to chew up your available bandwidth. I'd check your routing protocols and if one of them is very chatty. How are your routers sending and receiving route updates? if you send out route updates and receive them on your WAN link, might consider making the external side the passive side so that your router isn't getting route updates from other routers on the internet. If your router is always having to process route updates from other internet routers, that can reduce available bandwidth... Why have them when you only need to send packets not destined for the local to the next hop and let them worry about it....

I'd find out your actual utilization of your available bandwidth and what traffic uses the most bandwidth. It could simply be that the designed network and WAN link isn't sufficent....

without seeing some of the debug or packet capture on both inside and outside of the WAN link, there's not much to say except generalizations...

side note: if you think of ethernet and TCP/IP as the US postal service [or really any postal service], you will "get" what frames, packets / networking does.