it looks like the 1941 does not get the updates the 7600 is sending right after the neigborship is established. The 1941 brings down the session due to hold time expired.
You might want to check "show ip bgp neighbor x.x.x.x" on both sides while the session is up. There are a lot of counters that can give you an idea of what's going on. Look for "Message statistics" and also look at the bottom for "max data segment" negotiated. Try to to do a ping on the point-to-point link with different packet size to see if there is any issue with big packets.
In case there is a problem with the max data segment negotiated you can lower it on the 1941 with the command "ip tcp mss <#>" (global config).
also, migh not be related.. any reason why you're using "neighbor 172.16.140.21 transport single-session" on the 1941 ?