How fast are you raising the temp for your diacetyl rest? You may actually create acetaldehyde if you stress the yeast by raising/lowering the temp to quickly.
About 1-2 deg F per day is a safe bet.
Any yeast stress can yield acetalaldehyde - underpitching, under-aerating, temp. fluctuations, etc. Check all that out as well.
I've had this issue with my kolsch and found that I need about 1.5x the amount of yeast recommended by Mr. Malty. O2 and temp. control really helped, too.