...
>Since then, TSM Server stops at random times. (no messages in the actlog)

The *SM server has traditionally been programmed to produce an error log in its
server directory when it crashes.  Have a look for such.

You are hereby scolded for not including your software level in your posting:
it could well be that simply boosting your maintenance level will resolve the
issue.

A server with such issues needs to be carefully monitored.  In particular, watch
Database and Recovery Log consumption as the server operates through the day, 
and
watch for space constraints and anomalies.  Your surviving Activity Log from the
crash vicinity may record the start of some highly-consumptive process which may
need investigation.

  Richard Sims, BU