Welcome to Passionford

Regarding air leaks, if that is your problem, check the brake servo as these can leak quite badly, check all gasket joints for leaks, buy a can of carb cleaner or try WD40 and just spray along the joints..any leaks will show up with the revs rising for a moment as the leak/gap is fill with the fluid. Other than that its a little difficult to diagnose a fault like this on just a forum...hope it helps tho

Stu.