strange that its not popping a code. im not sure about obd2 but with obd1 there are some critical components that wont pop a code. not sure why but the tps and iac wont pop. of course without the tps it wont run but if its intermittent then it will run but run badly and will not pop a code. i would think they would have resolved this with obd2 but again i dont know very much (yet) about obd2 systems. imo look for those sensors that might not read and find out what the bench test values are and bench test them. like for example with my supercharged t-bird you measure the tps via vdc. perfect voltage at idle is .89vdc. with change in temps, barometric pressures, and altitudes this will vary around .05 +or- due to expansion and contraction of the alloys.