I tried to make a 64bit graph with the same results, Im using a Dell Power edge server R220 running server 2012 with a 10 Gig Broadcomm NIC with a 3 Gbps Fiber connection
when it goes over 1Gbps the graph starts over so 1.2Gbps shows as 200Mbps
as far as i can tell its snmp v2
Fix64bit was intended to overcome problems with 32bit counters overflowing near 100Mbps. 1.2Gbps is something else. First of all, check RRD Maximum for you 64 bit graphs, set it to something more than 10^10. Then add some more zeroes at the end just to be sure.
As for timeouts, if the results are same, it could be something else. Try running spine in debug mode (-V=5 for verbosity, -R for read only, -H=yourhostid). see what will it say. Try also running tcpdump -vvv host youhostip for looking into other side.
Working on these steps now to see what happens, It weird how it only messes up at the 550 mb mark
i added an image ..
both these graphs are polled from identically configured servers - dell builds both running the same NIC drivers
the only difference the server having issues has a higher WAN fiber pipe
i have a bandwidth monitor ap running on the server so i can see the server is using over 1 Gbps but its not graphing