So, it was twangy (requiring the services of a properly fitting guide) and the spring and seal are needing replacing. So a kit is required.

Some very good ones out there which will improve things - any of the following will be good - V-Mach, SFS, Tinbum, Welsh Willy, wonkydonky, Vortek. Listed in no order of preference.

For what it costs, replace the breech seal too whilst you're at it and bear in mind that not all of the kits include the piston seal.

A good de-grease, check, gentle polish and fitting the kit with the correct amount of lube will result in a much more nicely mannered little rifle. You were getting good results beforehand; it'll be even better once correctly fettled.