Thanks for your help I will check this out.Just some more information to add, this is an intermitten problem the hydraulics work fine but ocassionally this occurs when I am lowering with a heavy load form a height, it seem to be lowering to fast with a heavy load,also my machine has regen- lowering and when lowering only sometimes it displays on the BDI that the regen is working correctly.Also when the regen-lowering is showing to be working correctly the mast lowering speed is reduced which seems to be correct procedure.Why does the the regen-lower not be constant when lowering?could this be connected to this fault?

usually when there is a sequencing problem with the cylinders in the mast it is either something binding the channels or an internal cylinder valve problem.

check the mast closely and make sure all the rollers are ok, not broken or damaged and make sure the channels are clear of debris or any other obstruction that may cause them to hang up.

Then each cylinder will have to be removed and dissassembled and checked. Start with the center cylinder and once dissassembled check the safety valve in the piston and make sure it is intact. Do the same for the outside right cylinder also and if no problems found move to the left outside one.
In most cases i have found that the safety valve in the piston has come apart in the center cylinder causing the mast to malfunction and cause the exact symptoms you describe.