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Errors and Discard on Wan Link with QoS Applied

I have a number of remote sites connected to either T1 or DSL's which provide telephone service for those remote sites.  I have noticed after setting up Cacti/MRTG that I am getting a High number of Errors on the link.

WIll QoS enforcements cause errors or drops ?  Or is it more likely that there is an issue with the Circuit ?  
I notice that the errors seem to increase in proportion to CPU and Traffic Load.  We are still well under the link bandwidth max at all the sites.

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2 Solutions
Qos does not cause errors,
Errors can be for CRC's (medium/carrier) and Drops caused by misconfiguration
Run a Sniffer to see where's the problem.
-hope helps-
GerryDaltonAuthor Commented:
OK errors I kinda suspected were really errors, how about DISCARDS ?  

Well Discards occurs by a few situations:
- The router has access rules
- Virus/DoS/Boadcast/Multicast/P2P...Something is consume the routers/links resources (not just in BW also in number of packets)
- Firewall restrictions
- QoS Rules (priority packets are accepted over lower QoS)

>Check logs
>Use a Sniffer like tcpdump or whireshark
>Also check etherape and ntop.

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