Hi welcome to the forum
I'd be looking for a vac leak, fairly common with older cars. With all those years the lines would be dry and break easily.
Run the engine pop the bonnet and you might be lucky and hear the leak or even feel it by moving your hand around the engine bay, start near the throttle body.
If that does not resolve show anything a proper VCDS scan might shed some light on it or a smoke test would be another option.
MK4 GTI - Sold
MK5 Jetta Turbo - Sold
MK5 Jetta 2.Slow - Until it dies.
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