Deere 344J hydraulic issue

Just bought a 2009 Deere 344J for use on my farm. The loader has 4500 hours and appears to have been treated well in its previous life.

Since getting it in my yard last month an annoying problem has shown up. When starting out cold in the morning (been around -10C to -20C) the bucket cylinder operates erratically, if you start to dump it may drop uncontrollably and also when reaching the end of the stroke when curling the bucket back it sometimes immediately drops down. It doesn't do it every time you move the bucket but when it does there is no stopping the bucket drop from the pilot control handle. Usually by the time the loader has been used for 30 minutes or so the problem disappears and it operates as it should until the next morning.

The machine came here from the Southern US and the first day here I changed the hyd oil to 0W30, as per manufacturer's specs. As soon as I noticed this problem I disconnected the return-to-dig sensor as I thought that might be the cause. Did not make any difference.

The only other issue we are having with the loader is the bucket quick coupler seems to be seized, probably from not being used.

Any ideas on where to start looking?