Is it a folding prop? A non-geared folding prop will occasionally fail to flip both blades open, resulting in unbelievably violent 'vibration' as a horribly unbalanced (single blade, effectively) prop turns. Usually coming out of gear and re engaging fixes it. This is probably the only way a strong vibration like that could 'come and go'.
On one of our previous boats we got into the habit of a quick burst in reverse prior to engaging forward gear; that usually prevented this occurrence.
I doubt very much it had anything to do with your gland adjustment.
1984 Fast/Nicholson 345 "FastForward"
".. there is much you could do at sea with common sense.. and very little you could do without it.."
Capt G E Ericson (from "The Cruel Sea" by Nicholas Monsarrat)
Last edited by Faster; 02-28-2011 at 11:10 AM.