1996 Mitsubishi Pajero engine problem

About 2 months ago the engine in my NK Pajero 3.5L started missing and the tacho needle was flickering slightly when the engine light came on, my mechanic checked for trouble codes and showed 2 codes, crank angle and cam angle sensors, he replaced both sensors but after about 2 weeks the same problem came back, my mech. Took it to an auto electrician to check the electrics but after testing they said the electrics were fine but the timing belt was stretched so he replaced the timing belt, it was good for another couple of weeks before the same problem came back again, he checked all the earth points for good connection and replaced the sensor rings because one of the rings had marks on it like it had been in a vice but driving it home yesty the problem is still there, at the moment it starts to miss and jerk etc when the revs gets above 2000rpm. My mechanic mentioned trying another ecu. Do you have other ideas what it could be?