We started seeing that the mongodb clients are finding the primary to be down.
When i checked the mongodb log, i saw two different types of behavior/errors at different times:

1) simple INSERT into a (new non-existing) collection is taking very long time (~16 seconds) at times. The very same insert was running fine for years and nothing else changed

2) I saw the following errors:
warning: slow scan in allocFromFreeList (in write lock)
Sun Dec 10 13:00:07.698 [conn10363640] warning: newExtent 2223 scanned
insert xxxx ninserted:1 keyUpdates:0 locks(micros) w:10384820 10384ms

These two situations are causing an outage and should be fixed asap.

We are currently using 2.4.9 mongodb
I know that mongo upgrade is needed but this version was working fine till recently and nothing changed.
Any pointers will be highly appreciated at your earliest convenience.