Oracle memory gets trimmed every 6 hours

We have a very strange behaviour in our SAP R/3 Enterprise 4.7 production system (SAP_BASIS 620).

SAP runs on Windows Server 2003 Enterprise Edition.

10 GB RAM, PAE enabled (Physical Address Extension).

The affected server is the database server, which also runs some working processes (DIA, BTC and UPD).

There are also 6 Windows application servers (x32, x64 and Itanium).

After a normal SAP start, all Windows processes bit by bit allocate their memory.

oracle.exe starts with a Mem Usage 236 MB (VM Size 1.900 MB).

You can see this in Windows Task Manager.

After about 30 minutes oracle.exe reaches its average value of about 2 GB.

The value ranges from 1,9 GB up to 2,5 GB.

Then, about every 6 hours the following happens:

oracle.exe deallocates its memory completely !

No answer in SAPGUI, no reaction on the console for about 5 Minutes.

Then when i get the first look at the Task manager, i see that oracle.exe allocated about 80 MB.

In the next 20 minutes Mem Usage raises up to the average value of about 2 GB.

During this time, the performance comes up again step by step.

Not only Oracle is affected, at least every disp+work process also frees all allocated mamory.

But it seems as if Oracle would be the first to free up its memory and then drags down the SAP Kernel processes.

We have no changes made to the SAP Kernel, we did not apply any Windows updates.

SAP operated error-free for the last 2 years in this configuration.

The only thing we did, was to apply several SAP Support Packages (Basis + Application).

This behaviour occured the next day after we imported those packages.

So we have to suspect these packages, although the symptoms point to a problem with the SAP kernel, Oracle or the Windows memory mamagement.

SAP Support adviced us to reduce the load on the server, so we suspended some work processes.

Result: no improvement.

Next we reduced the Oracle cache size by 250 MB.

Result: the situation became even worse, the error occured every hour.

So we icreased the cache size up to 1,36 GB.

Result: could be an improvement, not sure yet.

I am wondering what must happen, that all processes on a Windows Server deallocate their memory.

Can a ABAP-Report provoke this error ?

Has anybody else ever seen such a behaviour ?

Any ideas ?