Access Violation c0000005 on Windows 2000


i've got the following problem:

my applications crash on Windows 2000 Pro with "Access Violation c0000005"

The apps are compiled with Delphi 5 (english, build 6.18 , Update Pack1)
System config: Windows 2000 Pro [multilanguage edition] (without SP1 yet)

The same apps run without ANY problems on Windows 95 and Windows NT 4.

The problem is very strange: when I open a ComboBox (there are no events on
it, like OnChange or OnCloseUp) the access violation happens. But it doesn't
happen every time, it happens "when it wants to happen" >> there is no
rule... if the access violation happens, Win2000 closes it and then, when i
start it again, it works (doesn't crash)...

Thanks in advance.