I'd suggest you post on www.vwvortex.com. They have a section specific to cabriolets. You'll probably have better luck there.

I have a '92 cabriolet. Your problem is likely a vacuum leak. There aren't a ton of vacuum lines, but you should check all the connections carefully to see if any are cracked. The lines themselves are plastic and shouldn't be a problem unless they've been crushed shomehow.

The extra fuel you get on a cold start is likely compensating for the vacuum leak and covering the problem until the car warms up and the fuel leans out.

Good luck.