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If you are going to work on a 964 at home you should definitely get a copy of Adrian Streather's 964 book. My 1993 RSA had the same problem as your's except it only has 11000 miles and the AFS was broken despite it running fine intermitently. I took it apart and checked everything and couldn't find any problems. After putting in a new unit (at the time it only costs $600) the codes were cleared and it ran perfect still today.
The K&N filter with the oils are not recommended for AFS (especially later 993 hot wire versions) in porsche models. The filter allows excess oils and dirt to gum up the AFS and hot wire leading to problems. Sticking with the OEM papaer filter and AFS has been just fine. Putting holes in the airfilter housing to increase air intake has no added benefits since the AFS barndoor is the main restriction to more air flow. The 993 on the other hand does benefit (about 8 HP gain) from opening up the air filter housing to let more air in quicker. The OEM papaer element is efficient and will protect your expensive engine much better than the porous K&N or after market stuff being peddled out there.
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