[93buicklesabrelmtd]

Buy a new vaccum modulator. Its aroud $17 dollars. I think the one Auto Zone sells is better than Advances own.

Make sure u have no vaccum leaks. Listen for them. Or spray brake cleaner on the hoses while the engine is running. See if u see a leak.
I would also change the fluid and filter.

You need to know or find out if your problem is electronic or mechanical.