Check the upper slide slipper bearings as kevin t suggests, just to make sure whether that could be a factor.
As far as adjusting the chains goes, you need will need to address any other issues (carriage side rollers and carriage main rollers) before doing any adjusting to chain length.
Carriages have two sets of rollers. There are "side rollers" which run PERPENDICULAR to the mast channel, and "main rollers" which run PARALLEL to the mast channel.
The small side rollers run on the web of the channel, and are intended to keep the carriage centered between the mast channels so that the main carriage rollers do not scuff against the flat plain of the web.
The main carriage rollers run on the flange of the mast channel, but very near to the radius where the web and flange meet.
Carriage side rollers are typically mounted on an eccentric axle to allow adjusting the clearance of the side rollers to the mast web.
Main carriage rollers typically use shims/spacers to adjust how they orient regarding their fit/clearance to the channel web.
Your problems be be due to everything, including the mast channels.......being "worn out".
Hopefully you will spot something that can be remedied, without breaking the bank.