system is loosing disk space at extraordinary rate

Hello all,
this is a strange one:
I did a release upgrade on a V4R4 machine (a 720) to V4R5 in november last year.
We were at about 50% used disk space at that time. PTF package C0198450 was
installed at that date as well, everything went fine.
Since we did not do any further IPLs since then, we someday noticed that 
something
is eating up disk space, until we reached 92 % last week.
Searching for the culprit was at no avail, we ended every user jobs, ended 
every non-system
job on the system, it eats space on and on. A short calculation led us to round 
about
300 MB (!) temporary space which is used up every day since november.
WRKSYSSTS showed 29 GB of temp used space out of 77 GB total, this is 38 % of 
the
whole system !
When we scheduled an IPL to get rid of the temporary used space, I did endsbs 
*all *immed
at first hand and suddenly all our used disk space was free again ! You could 
watch it coming
back available when pressing F5 in wrkssysts display. I never saw this 
behaviour on an AS/400
before. IBM support was nearly as helpless as we, we ordered some PTFs that 
might help ...

Any hints anybody ?

Have a nice easter weekend,
regards from germany, Philipp Rusch