1990 Mk1 1800 Golf, Gearbox problems

Recently bought this car and had no problems until recently. Gearbox keeps grinding into third at high rev but slides in seamlessly under 2000rpm. Also grinds from first to second or third to second but only when cold ??? Any ideas or advice ?

First off I would try make sure gear linkage is adjusted properly with no play whatso ever - if there is change the bushes and linkages, and also change the gearbox oil for some fresh and of the right quantity.  This should improve things, but the syncos in the gear box will probably be worn by now