The truck is a 1994 GMC Sierra 1500 truck w/approx 170K miles on it.

I would appreciate any feedback on my problem...
I have had emissions problems for a few years now as can be seen in the following emissions transcript...

Emissions Inspection 6/4/2002
Hydrocarbons (HC) – Inspection 5.3095 – Requirement 2.7000 FAIL
Carbon Monoxide (CO) – Inspection 35.3126 – Requirement 60.0000 PASS
Oxides of Nitrogen (Nox) – Inspection 0.4853 – Requirement 5.7000 PASS
Carbon Dioxide (CO2) – Inspection 476.3944

Work Performed 8/2002
· Adjusted Timing to 0 TDC
· Replace Oxygen Sensor @ Catalytic Converter
· Replace Spark Plugs
· Replace Ignition Wires
· Replace Distributor Cap & Rotor
· Replace PCV Valve
· Replace Air Filter

Emissions Inspection 8/2002
Hydrocarbons (HC) – Inspection 0.1880 – Requirement 2.2000 PASS
Carbon Monoxide (CO) – Inspection 1.9014 – Requirement 60.0000 PASS
Oxides of Nitrogen (Nox) – Inspection 0.0070 – Requirement 4.2000 PASS
Carbon Dioxide (CO2) – Inspection 73.1697

Emissions Inspection 7/13/2004
Hydrocarbons (HC) – Inspection 6.7361 – Requirement 2.0000 FAIL
Carbon Monoxide (CO) – Inspection 62.4512 – Requirement 60.0000 FAIL
Oxides of Nitrogen (Nox) – Inspection 0.3481 – Requirement 4.0000 PASS
Carbon Dioxide (CO2) – Inspection 599.9200

Work Performed 11/2004
· Adjusted Timing to 0 TDC
· Replace Spark Plugs
· Replace Ignition Wires
· Check and Clean Distributor Cap & Rotor
· Replace Air Filter

Emissions Inspection 7/13/2004
Hydrocarbons (HC) – Inspection 4.6401 – Requirement 2.0000 FAIL
Carbon Monoxide (CO) – Inspection 18.6279 – Requirement 60.0000 PASS
Oxides of Nitrogen (Nox) – Inspection 0.2441 – Requirement 4.0000 PASS
Carbon Dioxide (CO2) – Inspection 609.0446

Still notice low throttle, low RPM miss (seems to get a little worse as engine temperature increases). No miss upon acceleration or at high RPMs that I can notice. After engine warm-up and when truck is parked, holding the accelerator slightly so that engine revs around 500 – 700 RPM (approx), the engine will stall, revs will decrease, engine will recover and surge back to 500 –700 RPM, stall, and repeat in a cyclical manner.

Work Performed 11/2004
· Check and clean EGR valve. Valve seemed to be fine, but cleaned.

I'm hoping that someone can give me an idea as to what could be the cause of my problems...I'll gladly reply if further info for diagnosis is required...Thanks