Is that SM really at 38km? If so, then I don't see how you'd be getting that RSSI at that distance. The signals are too strong and are overloading the radios. What happens if you turn down the power on both ends till you get an RSSI in the 50's? Are you using 3.5.2 on all radios? Are you using large parabolic dishes on these radios?
It -should- tell you under the SM's "Wireless" tab why it is disconnecting. You may also find the reason in the syslog of the devices. As Eric stated, your RSSI in relation to distance is not correct. Yor SNR values are low in comparison to RSSI. You may need to run spec-an as well.