The interesting things I see about the configuration is the half-duplex setting on the F0/1 interfaces (make sure you don’t have a duplex mismatch), neither side of your T1 is injecting timing, and are you certain it’s sf and not esf framing on the T1?
Check your packet size to ensure that they aren’t too large and require fragmentation. Also make sure that the Don’t Fragment bit is not set on either the client or server side. You may have to change your MTU size and mangle the packets to remove the DF bit if this is your problem.
Finally, have you checked your Serial interface counters for interface resets and carrier loss? You might have a problem with the circuit.
Hope this points you in the right direction.