Hi Ashish,
Are you aware of the rate of traffic going through the link, when you see increase in overruns?
Overruns indicate that the rate of packets coming on the interface is so high, that the ASA is unable to pick the packet off the wire.
If that's the case then you would need to see if that rate of traffic is normal. If it is then you might need to divide the traffic somehow between 2 interfaces.
Please attach the output of "show tech" if possible. The block usage and traffic statistics along with the config might help in determining why there are overruns.
Also, please check that the speed and duplex settings are hardcoded rather than set to auto on both ends.
-Shrikant