check the gear linkage for any worn bushes or pivots as any lost motion means reduced effort at the gearbox and replace as necessary. Ideally have your assistant move the gear lever with you watching the linkage and see if you can spot where there is any excessive play or lost motion.
then check along the cable lengths for any damage or signs of heavy wear / fretting, if you find any replace the cable. There may be some adjustment on the cable tension at the gearbox end (depending on the year) and the cable should be tensioned so there is a minimum of slack but not so its tawntThen check gearbox oil levels and if its not been changed in the last 20K miles then drain it off and fill with fresh. I can strongly recommend the use of Castrol Syntrans 75W90 (was called SMX-S) as I've known it help with shift issues on many manual gearboxes
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