I had an 8v golf gti that had the same scenario you describe. I think the 1600 lump is an older VW engine and is probably quite similar the my gti. The idle contol valves tend to gunk up with oil sludge, give it a good clean with carb cleaner, also clean the throttle body. This gets a carbon build up which stops the butterfly closing and allows air to be drawn through it, bypassing th idle contol valve. It could also be an air leak, check all the pipes after the MAF and look for splits in the vacuum hoses,
