I'm using 3845 router with ATM card. I have a few 10Mbps PVCs, and a few small speed PVCs - for example 512Kbps. I observed congested high speed PVC impacts low speed. Low speed PVCs have output drops and big delay with transmit packets.
The problem is likely that packets destined for the congested PVC uses all of the buffers on the tx_ring, so packets destined for the other PVC's either gets dropped due to no buffers available in the tx_ring, or have wait to get serviced until all the packets going to the congested PVC gets sent as the tx_ring is a FIFO buffer.
I don't recall what the command is to configured a limit on a per vc basis, it might even be hidden. At my previous employer we used this on 7200/7500's with PA-A3 cards to solve this exact problem.
If you open a TAC case they should be able to help you.
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