If it has a leaf-shutter your problem is with the retardment part of the shutter.
The way to go:
Remove the frontlens
open the shutter-compartment (usaly a ring holds it together)
remove the reatrdment (looks like a small banana) and soak it in gazoline for a wile until it runs smoothly again.
dry and lubricate with synthetic oil and remount everything.
This is the way to go with copal and compur shutters.
At times however the retardment is adjustable, in that case you need a shutterspeed-tester for propper adjustment,
otherwise you will be out of 1/15th and a second will no longer be a second, but more or less.
I've been servicing my own 4x5" lenses for the past 30 years and I have a tester.... location: Europe.
The best thing you can do is to go to a local repearman that you trust, the repair will take about 1 hour.
Good luck,
archphoto@aol.nl