Tracing Or Debugging A Procedure

I have a very simple piece of code (see below) which when executed sometimes takes around 7 minutes and sometimes around 3.5 hours. the difference is that during the 3.5 hours there is a lot of querying of the table being updated. But I don't know how to find out if this is the case. How can I find out whether my process is waiting (for locks or for any other reason) - is there a trace or debug facility within the tandard Microsoft Toolset which I can use.

Regards
Colin

Problem code below
===============
print 'Updating stm_brnline - Start time is ' + convert(char(25),getdate(),113)
--
update m
set m.branchpgrade = s.branchgrade
from stm_brnline m, tmp_brngrades s
where m.traddiv = s.traddiv and
m.contcode = s.contcode and
m.merchsect = s.merchsect and
m.branch = s.branchcode
--
print 'Updating stm_brnline - End time is ' + convert(char(25),getdate(),113)