i am not sure what the solution or what is the cause of this problem
but it could be the clock rate is not correctly set
or there are protocol mismatch such as one could be running HDLC and the Other could be running PPP.
but if the link comes up and then down after a while and up again the cable could be faulty.
again all of what i said is just an assumption.
if you need accurate solution then you may need to post your configuration on both routers
some show commands as a reminder.show run
show ip route
wish you all the best