The car sputtering and dieng could be a fuel issue. Gasoline breaks down over time and can clog up lines and other components. Two possible things that come to mind though is your fuel pump and fuel filter. The fuel filter may be clogged or the fuel pump might not be working properly.
One thing to try is if you have the abilities to do do is pump the fuel out of the tank and replace it with fresh gasoline. If you can't pump it out that's fine too. I had to start a GMC that had been sitting for two and a half years and we couldn't pump the gas out. What we did was add 5 litres of new gasoline and an octane booster. After we added the gas we kinda rocked the truck a bit to help it mix. It started up fine which was great, so then we let it run for A while to help get rid of the old gasoline. It's been running fine ever since.
I'm not familiar with the fuel filter and fuel pump replacement on a Z3 but I'm sure I can help you find it if what I mentioned above doesn't work.