1982 Range Rover Classic

I own a 4 Door, V8 3.5 petrol engine, with twin S.U.'s, a 4 Speed Manual Gearbox. My Range Rover has just passed its M.O.T. but please can anyone help me with advise? The v8 engine keeps going out of tune every few days and it sounds rough! whils driving! I have not got around yet too doing an oil change! the timing seems o.k. But I really think that the last owner did something with the Distributor! I try my best at keeping the engine in tune! I do have to check the whole exaust system as it was mentioned by the MOT tester to get it checked as it is corroded. I'm doing my best at restoring the range rover, but on my meager money its hard!