i have exactly the same problem (except my car isnt the same) mine is a xr2i running a tall block conversion , i dont know how ur car engine bay is laid out but on mine the starter is close to the manifold/downpipes and i have been told the heat from the exhaust affects the performance of the starter motor, could that be an issue with urs, i have just ordered a starter motor heat shield to see if that solves it??