This is a common problem due to gear slipping on shaft. Do a search on the forum and you'll find a variety of approaches of how to fix that ranging from glue to crimping shaft.

Been there, done that, on my old 250C too

Good luck,

Bert

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'70 111 280SE/c 3.5 (4 spd manual) - sold
'63 MGB
'73 MGBGT V8