Took my polarizer to the Canon service center and asked them to clean it for me again. They claim not to know why this is happening but said they have seen this before.
WHen I asked how they cleaned it so I could do it myself they told me they used Windex followed by "100%" alcolhol. I'm not sure where you'd get 100% alcohol.
1. Another customer behind me at the counter said she had once inadvertently disassembled her polarizer and it was full of oil/lubricant (see howiesmith's post above under this topic)
2. At www.cleaningdigitalcameras.com
while browsing their site about sensor cleaning I came across this description of how Canon cleans the sensors and it mentions Windex and 90% isopropyl alcolhol: Canon - Blower and a Kimwipe, held by tweezers. They do not like to use fluid but when necessary they use either 90% isopropyl alcohol or a 50/50 mixture of isopropyl alcohol and Windex.
"Windex" is mostly water with alcohol and amonia added along with some water softener, coloring and perfume. The amonia is added to make it smell like it works and the perfume to help cover up the amonia. You can make your own, cheap.
"100%" alcohol probably means you can't drink it and it doesn't have stuff added. Rubbing alcohol usually has glycerin added to help lube the rub.
Just make sure you have stuff that won't leave a residue when it dries (distilled water and alcohol) and use a cloth that "won't scratch" the filter. I usually use a "glasses cleaner" made by Zeiss. Premoistened paper wipes that "won't scratch." Seems to work well, but I don't use them on really expensive things, like Zeiss lenses (except my glass which get replaced frequently). Camera lenses get a shot of clean air (oil free compressed air) and a light brush dusting. Heavy cleaning usually isn't required and gets done when I have the shutter timed and cleaned. Filters I place in the consumable category.