If you mean it jumps out of gear, at that age it's very probably simple wear. You need either new gears, to have a gear-cutter make and harden new ones, or get hold of a good second hand gearbox. It won't be cheap, but in those days a lot of parts were interchangeable (no computers etc. to be a nuisance!). Best of luck finding one.
Leedsman.
p.s. I assume you've checked the correct oil is in use, and the correct level? People put ATF fluid in manual gearboxes -- not a good idea at all. Esp. the old stuff.

 

P.p.s. Can you post a picture of your old Jag??? Fully restored, it could be worth a lot of money.

 

A quick search turned up this firm.

http://www.worcesterclassicspares.com/