A bit difficult to diagnose without being there - but here is a few things to be looking at:-
1. have a look at the routing table in both routers to see if there is a host route to the neighbor IP address being learnt from somewhere else.
2. Do both routers have complete arp entries and are they the correct mac address.
3. Are there more that 1 address (Primary/secondary) on the ethernets and are they both primary
4. Are you running HSRP between these routers - I have seen some strange results ping HSRP addresses.
Hope this helps a little.