Acrobat 6.0.3 (OS 10.3.9, dual G5) flushes Acrobat Distiller 4.05 PDF files that contain CMYK or spot color gradients created in Illustrator 8. The same PDF files are printed without a problem from Acrobat Reader 4.05a (OS 9.1, G3). The gradient uses five control points; have not tried fewer control points. The spot color version of same gradient uses only one PMS color at different tint values (outputs on imagesetters without problem). Converting to CMYK makes no difference (still flushes). Optimizing PDF file for Acrobat 6 compatibility makes no difference. Cryptic message about image mask appeared once (OS X), but didn't note what generated it at the time (Acrobat?). Printer is HP Laserjet 4M (Adobe Postscript Level 2), which has served well as "acid test" for PS printing for many years. Haven't tried resaving gradient in Illustrator CS. Variables are multiplying like rabbits. Can anyone shed light to steer experimentation?