What kind of issue do you have? packet lost or only no db conectivity?
If the problem is connecting Oracle database, listener is always aswering? What about load average, disk usage etc... just this time?
could you take a look if interface is overloaded?
Tell your networking boys get some use stadistics of ports or even check the physical wire.
When this issue appears, all boxes of this subnet is working fine?
If I have more ideas I will let you know but this is very rare