The "line"?

There are many many vacuum lines on the 2.6. Given the age, I would start by replacing each and every one of them. Many of them you cannot see. I don't know what you checked, but I have a feeling you were not able to get to every single one.

I would also check the intake tube from the MAF to the throttle body. Look for cracks in the accordion section, especially on the underside.

Do not adjust the throttle stop. That is not how you adjust the idle on the 2.6. It is done via the idle air bypass, but you need to address the underlying issue first. What you are describing is not an idle setting issue.

After that, go back to the basics. Check the timing marks, adjust the valves, set the timing. You can also check the TPS. It has specific settings that need to be checked, especially if it has been messed with.

Good Luck,
Michael