Scheduler appears to be hung ...

SQL 7.0 SP4 logging this message :
The Scheduler 0 appears to be hung. PSS 0x652E50D0, EC 0x652E5280, UMS Context 0x2FB7E0A0

While SQL is logging these messages SQL is blocked
This is hapening 5-10 times in week.
In traced file I found that there is no same query from application that causes this blocking.
What this qould be ?!
Is it SQL engine or is it hardware on underlaying-server ?
What with combination beetwen SQL 7.0 and Windows Server 2000.
Or even is it problems with queries from application ?HTH

http://www.eweek.com/article2/0,3959,920568,00.asp|||Thanks for linksql