is your idle speed ok in general?do your brakes feel ok?is there any unusual hissing sounds from the drivers footwell area?
check-speedo cable sensor(instrument panel end),idle stepper moter operation,has the stepper moter got the additional signal amplifier on it-a small black unit plugged in between the stepper moter and its connector,brake servo for air leaks and a general look at the breather pipes on the back of the manifold.