WinXP, Filemaker 8.5 Adv

When working with fm I experience strange keyboard trouble. Especialy during
field definition, but sometimes also during normal use. It all starts good
and I start working.
but without notice some keys of the keyboard suddenley display different
signs. They just change. What
used to be a * now becomes a (.
The ( becomes a ),
{ becomes <, etc etc. All
normal letters remain normal. The only remedy seems to shut-down
filemaker.exe (not just the file)
and re-open the file. Sometimes it happens after an hour, but sometimes also
after five minutes. (And then again after a couple of minutes). Generally I
have no other programs running (exept the system ofcourse). But when I try
any program simultanously
these don't have the same trouble. So it seems filemaker only. Any idea's?

Ursus