One conclusion is that the cap does not absorb enough IR, the other is that IR through that port does not cause the issue. Does adding extra IR-blocking material, like tape or the alufoil Hero999 mentioned, help? Maybe there's another path for the IR to reach the sensor, or it's something else like heat. Do you see any difference, like sensitivity or time until the problem occurs? If there's no difference at all, I wouldn't expect adding more off it to help.