Not a solution, but a suggestion: Send this to Hauppauge:
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Dear Sirs,
I have recently purchased one of your Hauppauge WinTV PCI Tuners, which is not functioning properly on my computer, which is running Windows 7 64-bit with <insert amount of ram here> of RAM.
I previously attempted to get assistance from your company, but was told that my system actually had too much RAM to work with this tuner, and was advised to remove it in order to solve this problem. This is no excuse for having received a device which does not work the way it was advertised. I will ask once more whether or not there is anything that can be done to rectify this situation. If you are going to tell me that my computer's hardware setup is the problem and that your tuners only function under very, very specific scenarios, then I will be taking my business elsewhere for future needs.
Thank you.
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Companies don't typically enjoy hearing from me, as you can probably tell.
As for an actual suggestion? I really don't have one. I can only think of what is probably causing this, but as far as I can see, there isn't any real way to 'fix' it. I'll look into it for you, though, and keep you updated if I find anything...