Failure of Emissions State Inspection

2001 4 cylinder, autmoatic with a 2000 engine from Japan. Odometer-116,000. Engine put in in 2005 had 30,000 miles. I continue to fail the emissions test for my state inspection (Texas). Check engine light will not stay off after clearing codes (after repairs have been made). It returns within 10 miles. Following repairs have been made PRIOR to test.
1. Catalytic Converter (twice in four months)
2. EGR Valve
3. Ignition coils (twice in two months)
4. Ignition wires
5. Spark plugs (twice in one month)
6. Idle adjusted (to correct horrible idling problem)

What am I missing? My mechanic has to call in a mobile diagnostic unit because they can't pinpoint the problem and the check engine light continues to come back. I've spent $1500 trying to fix this problem.