Here are some pictures of the "splotches" (click the image to advance through the pictures).  They can range from small specks of paint to splotches that are a couple inches wide or more.  I'm still troubleshooting, but any advice or suggestions would be greatly appreciated!  Below are some of the areas I'm troubleshooting if it helps:

• Paper:  Using Rives BFK, which is often recommended among gum bichromate printers.  I can test another paper, but I sure like this stuff.  It dries flat, hold up great to multiple washes, doesn't stain, shows good detail... I'm currently working on large 40"x50" prints, so I need a paper that is available in a large roll.  I sincerely hope the Rives paper isn't the problem.
• Paints:  It happens on multiple paints, so I don't think that's the culprit.  I use mostly Schmincke and Sennelier paints, both of which are recommended by various bichromate printers.
• Casein:  I'm using the ammonium caseinate available through Photographer's Formulary.  I'm going to use the cottage cheese method this week and see if that gives the same results.  I have found that image quality goes way downhill if the casein gets old, but I get the staining problems even if it's fresh batch.  I also tried lengthening the time I run it in the blender to make sure it's fully mixed.  Still got the same results.
• Dichromate:  I'm using a 10% potassium dichromate solution.  I've tried mixing new batches (although I'm not aware that it ages) and it didn't seem to help the splotches.
• Sizing:  This seems like a good suspect, but it's odd that the stains happen in different spots in each color layer.  If it were a sizing problem, I would expect each paint layer to affect the same spot.  I'm currently sizing with gelatin, then doing a separate formalin hardening bath.  I'm experimenting this week with two layers of gelatin sizing just to see if the results change.  I've used glyoxal and formalin with similar results.
• Painting technique:  It's occurred to me that maybe the paint soaks into the paper if I coat it too thick.  I'm using a bristle brush to apply the paint mixture, then using a foam brush to "squeegee" the mixture into a thin layer.  I've gotten the best results with a thin layer of the paint mixture.  The 40x50 prints are challenging to coat, but even 8x10 prints exhibit the same problems.

If you think of anything I might be missing (or if you've experienced similar problems), I would be thrilled to hear about.  Thanks again for your time!