the carriage stop may be contacting the second rail. sometimes the p/m tech's will overadjust the freelift chains and this exact problem occurs. eventually it will break the stop off of the rail. there was a problem with the 6 series units mast geometry and adjusting the forks to be off the floor when fully lowered required shimming the secondary cylinders. there is an old service bulletain on this issue. im sure you've already checked this but just in case.