Sounds engine running condition rather than gearbox

get a service

replace spark plugs , engine oil/filter , fuel filter air filter adjust the timing if the distributor with a timing light/ gun. Run some 95 or 98 octane fuel see if that rectifies the issue.
Also try not to overload your engine try to keep in a lower gear for alittle longer. You are lugging the engine and possibly damaging it by apply too much load at thenwring speed/gradient for the gear selected