Hi its the clutch master cylinder holding off, I had it on my car they are Cr@p plastic thing and the seals swell if the brake fluid is a bit past its change date, mine went about 3 months after I had my clutch changed the reason the symptoms are random is because it is all regulated by the torque bite depending which gear you are in say 3rd at 1500 rpm as soon as you push the throttle as the torque increases the the thing will briefly slip then grip, do that for too long and it does wonders for the driven plate wear, quite a bit of stripping down under the plastic scuttle to get at the cylinder and about two hours graft in all, did mine and used an O.E. cylinder from TPS as my mechanic told me the Febi units were failing quite frequently, bled everything with fresh fluid and not had a problem since, BTW Ford are having a real problem with this with Transits and now insist you change the clutch master when you put in a new clutch.