What have you tried and most importantly what engine?
I have NEVER had an ISV go wrong and i've been through a lot of golfs. They rarely go wrong.
I think I have seen you post this before. Which, along with cost, is why I left replacing the ISV until last. This is not a comprehensive list, I can't remember everything I have done to try and solve this. But:
Checked and replaced all vac lines and blanks.
Cleaned the TB/inlet
Replaced the air take ducting (old one was split)
Cleaned the ISV completely
Replaced the rubber bung on the rocker cover
Followed rubjonny's set up guide
Check that the idle switch clicks and is operational
Blue temp sensor
EDIT: Sorry, '91 8v