Hi Joel,
what's usually overlooked is that traffic can be very bursty, though looking at the show interface output rate you may just see 50Mbps.
My questions are hence:
1) how you measured the output rate? Just with the interface counters or you use some traffic generator?
2) what kind of link is that? Is a provider limiting somehow the allowed throughput?
3) is there any policy-map or rate-limiter applied to the interface?
4) is most of the traffic TCP or UDP? In the former case you may configure some congestion avoidance mechanism (WRED)
Please have a look at the following public documents for more pieces of information at regard:
http://www.cisco.com/en/US/products/hw/routers/ps341/products_tech_note09186a0080af893d.shtml
http://www.cisco.com/en/US/products/hw/routers/ps133/products_tech_note09186a0080094791.shtml
Regards,
Antonio