i'd recomend a service,
some fuel addative too
if car has been stood maybe jus stale fuel, if this doesnt cure the problem, it could be down to a lamba sensor not working properly. if the ecu thinks the lamba sensor is working fine it wont show any faults,

does your OBD4000 show real time running??
if so drive down road with it plugged in and check peramiters of things i.e lamba sensor
throttle postion
things like that

does it hold back on power at all? i.e struggle to rev sometimes?
if it does then could also be related to lamba sensor, or maybe somthing alot simpler i.e knock sensor