First thing that I would do, is inspect all of your vacuum lines...make sure they aren't cracked/broken. Physically inspect them when you can reach them...bend them, tug on them, make sure they are still good.
Next thing (that would involve no $) would be to check the cap and rotor...clean the contacts if they look groady...(if you notice any improvement that could very well be your problem). After the cap and rotor check, inspect the ignition cables themselves...I've read that you can spray a mist of water on them at night, and observe arcing if they are in need of replacement.
After that, you can spend a few dollars on a fuel filter, and see if that lends any improvement.

OH! Check the timing...It just occured to me that the timing chain rails might be original...and the possibility that something is awry is there. Do a search on this subject (make sure you have a little time). If you should find out that the chain and rails have already been replaced, check the timing again to make sure it was re-assembled right. Is the "Economy" gauge (aka vacuum gauge) giving you any hints? ie: when it idles, where is the needle? when it is "stumbling" where is the needle?

Good luck in locating your troubles!

-Larry

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It is a truism that almost any sect, cult, or religion will legislate its creed into law if it acquires the political power to do so. Robert A. Heinlein


09 Jetta TDI
1985 300D