Input queue: 0/75/2498545/1023776 (size/max/drops/flushes); Total output drops: 74
Queueing strategy: fifo
Output queue: 0/40 (size/max)
5 minute input rate 252000 bits/sec, 147 packets/sec
5 minute output rate 87000 bits/sec, 21 packets/sec
2230357044 packets input, 3275392715 bytes, 1093734 no buffer
Received 530570600 broadcasts (0 IP multicasts)
0 runts, 0 giants, 223188 throttles
633215588 input errors, 0 CRC, 0 frame, 633215588 overrun, 0 ignored
0 watchdog, 0 multicast, 0 pause input
3125809638 packets output, 1809711886 bytes, 0 underruns
0 output errors, 0 collisions, 3 interface resets
13700099 unknown protocol drops
Most of the above bolded stats show your router cannot keep up with the ingress rate. The unexpected, to me, stat, is the high number of unknown protocol drops. What's on the other end of that link (a switch)?
Although Cisco recommends a 1921 for WAN circuits of only 15 Mbps, that allows for pretty much the worst possible case. Depending on what your traffic is comprised of, and your router config, up to about 100 Mbps (aggregate) throughput isn't impossible. So, ideally, would like to better understand your traffic mix and your router's configuration. Without knowing, difficult to propose things to do.
Possibly, increasing the input queue depth might help.
A possible immediate work-around "fix", would be to run that interface at 10 Mbps or have other side "shape" its egress to less than 100 Mbps, both only possible if other side supports.
Reviewing your buffer stats, might show buffer tuning might be helpful too (or, if not already using, and if your IOS supports, enable auto buffer tuning).
Again, though, analysis of your traffic and your config, would be the ideal next step.
BTW, what's your CPU history stats look like? (Edit: I do see you mention not having CPU issues, but I wonder about 100% usage spikes.)
