Problem starting

My 2003 Mazda Protege5 often has difficulty
starting, though if I stomp on the clutch hard
several times or wiggle the clutch while turning the
key, that usually does the trick. I've also had
success by slamming the hood, and it has less of a
problem when parked on a downslope. The battery,
wires and starter have all been replaced but made
no difference. Ideas???