Zacks' is great. I have sent a few lenses to him and he did a great job. So does Carol Flutot.
One option to consider is to unscrew the lens elements and soak the shutter in 95% alcohol for a few hours, working the shutter now and then. Usually, the shutter will fire more normally as the alcohol loosens up old lubricant and also acts as one. After the alcohol soak, put it in naptha (white gas, Coleman fluid) for few minutes. This gets rid of the alcohol and it dries fast. I have tried just the alcohol and after it evaporates, the shutter will sometime be slow again. Years ago I sent a shutter to Carol Flutot and she mentioned that she soaks the shutters for days before working on it.
After this treatment I use a home-made shutter speed tester and mark the actual speeds somewhere on the camera. Usually, the slow and medium speeds are close, but the faster ones (like 1/400th) could be a stop slow. But that's a spring strength problem. I hardly use speeds faster than 1/100th anyway.
I have done this with several lenses, for 4 by 5 and 2 by 3, and use them a lot with no problems. I have sent three of them out though that were actually broken, not just sluggish.
Good luck!