Cisco 1721 Router and X21 Frame Error Problem

[jagotm]

I have a two Cisco 1721, with WIC-2T cards, which are setup to communicate with each other over as ISDN B channel using HLDC.

I can ping either end but the performance is terrrible. Typically I am losing between 40 - 60% of packets.

When I do a show 'interface command' for the serial ports the input errors are astronomical. apart from a few CRC errors nearly every frame has an error.

I have tried tx clock inversions but that dosen't seem to help.

Could anyone point me in the right direction.

More info if you need it, from memory

Router 1
fastEthernet IP 192.168.1.1 255.255.255.0
ip classless
no ip domain lookup

Serial0
ip address 192.168.10.10

router rip
192.168.1.0
192.168.10.0

Router 2
fastEthernet IP 192.168.2.1 255.255.255.0
ip classless
no ip domain lookup

Serial0
ip address 192.168.10.20

router rip
192.168.2.0
192.168.10.0