Unusually High CPU On SQL Server The 2nd Day After A Reboot.

To All,

I have a SQL2KSP3a database(<1GB) running on a 4x3GB physical CPU with
4GB of ram. It is Windows Server 2003 with hyper-threading turn on.
There are ~420 .Net users/cxns (fat client, no web/app servers) with
connection pooling and ~1 trx/sec. The database growth is neglegeable
and actually is not even relevent which I will explain in a minute.
99% of the trxs are from one SP that does a select. The resultsets are
relatively small as well 1~100 rows. Yes I have tuned it with index
tuning wizard as well, changed the SQL memory configurations, etc....

My problem is this...
The first day after a reboot, the server runs 6%CPU during peak hours.
During the non-peak hours until the next day something apparently
happens. The next day (2nd day after a reboot), it jumps to 40%CPU
during peak hours. The server will continue to run at 40%CPU during
peak hours until the next reboot. This phenomenon has been occurring
for 6 months or more and the traffic on the server is the same for day
1 as it is for day 2,3,4,... This database was on another server with
100+ dbs and exibited the same behavior, thus bringing that server to
its knees, and thus we had to move it to the server in question with no
other dbs.

I have googled my eyes out, Microsoft site, white papers, perfmon,
SQLDiag, PSSDiag, execution plans, index tuning wizard, and the list
goes on! I currently have a case open with Microsoft that has been
open for months now. I have been passed around to the 3rd "MS Tech
Specialist". I have ran PSSDiag a total of 6 times for them for hours
on end. I have changed MAXDOP. I could give more information, but I
would be here for days. I am running out of patience/ideas and
Microsoft is apparently blowing smoke.
Any ideas are greatly appreciated!
Thanks in advance!
JL