Chasing "signal 11" issues

Since Monday I have been seeing "terminated by signal 11" messages in my
7.4.6 + Slon 1.0.5 system,. but only on the master

I've done a dumapall, initdb and restore , which reduced the frequency but I
still get them 6-8 times a day.

After turning up logging it seemed to die when calling a very small table (2
rows, 4 columns, 8 char text strings), but manually selecting caused no
issues, so I then took a hit and shutdown the system and swapped out the RAM
(from earlier list suggestions).

This seemed to work until 7 hours later when the problem has reappeared, at
a higher frequency too.

It is ONLY occuring on the master, not on any of the leaf (replicated)
nodes, and seems to be triggered by a few different systems connecting (so
no common code base)

Suggestions/help ?