Incorrect totals the first time a report is run

NAV5SP1 – build 27191, SQL2005 SP3.

I have the strangest result EVER with a report in NAV!
It turns out that the first time I execute it, the calculated totals (from CREATETOTALS) are incorrect. After that it keeps doing it right. Each time I re-open the database the problem comes back, but only for the first run. I have tried it 30-40 times and it is always the same. The first time it seems to miss one of the first entries in the calculation, but after that it works like a charm.
It is an old homemade report that is going through some entry like records with a filter on a Journal No. field. It has been used for years in Native 3.70 without any problems, but now the problem suddenly appear after migrating to NAV5SP1-SQL. The problem doesn’t exist for all journals, so sometimes it gets it right the first time as well. The problem also disappear from the journal if I re-open the database and run it on one of the journals that isn’t causing problems. So it is not a question of having read the actual data before or not.
I’m currently trying to re-produce it in native or make the error appear as simple as possible in SQL before reporting it. But has anyone seen this kind of behavior before?