Hello and Welcome to justanswer...
Im sorry for the wait on your question..
On these models, the usual
fault , is indeed with the cables -
I dont know how they have been checked - but to rule them out completely, you need to disconnect one end - transmission end.. then see if the lever is still stiff. If it is, then its either the cables or the linkage at the base of the lever.
If its not , then the cables are at fault. Too - you can try operating the gear change manually - on the trans. You should be able to select gears relatively easy that way... If not, if its very difficult, then it would seem you have an internal problem on the transmission.
Not a really common fault as I say, its normally the cables etc - So do check it as above before doing much else to determine which as at fault.
Im very pleased to have helped you -Let me know if I can help anymore...
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Kind Rgds - MIKE