Here's my take on this, the Howell TBI is limited since the system is stripped down to bare bones sensors and does not control the ignition.
The questions I have are this
1. Are you running the stock heated manifold?
2. What is your ignition timing set at idle?
3. Have you removed the TPS or IAC?
Castle Rock Colorado is higher than 6000 feet above sea level and I don't believe the map sensor is parametrically compensated.
So given the information you described I would look at a couple things.
One check for vacuum leaks and make sure that you do not have any. 90% of the Howell problems are caused by vacuum leaks.
Two have you played with TPS or IAC if so you need to reset those and here are is how it's done courtesy of third Gen.org
IAC and TPS Adjustment
Tom Keliher Mar 31 2006 - 3:07pm
Idle Air Control
Torx bit # T-20
Take the paper clip and open it up and form it into a big "U" shape. Insert the clip ends into the ALDL in the 'A' and 'B' pins.
Turn on the ignition, but don't start the engine. Wait 30 seconds. Now, go remove the connector from the IAC.
Start engine. You are now going to adjust "minimum air". There is a Torx screw on the side of the throttle body. This is what needs to be turned to adjust minimum air, or more commonly known as "idle speed". It comes from the factory with a protective metal cap over it. If the cap is still there, use a small punch to knock it out. Set the idle speed to 450 rpm, rotating the Torx screw clockwise to raise rpm, and counter-clockwise to lower rpm. Once the idle rpm is set, turn off the engine.
Re-connect the connector onto the IAC. Start engine. Idle speed is now once again governed by the ECM, but your idle should be smooth and steady, approximately 600 rpm in Drive (for unmodified cars).
If you set an SES light by having the IAC disconnected, then after shutting down the engine disconnect the negative battery terminal. Wait 5 minutes. This will clear the ECM of all trouble codes. Re-connect the battery and drive the car for 20 minutes to allow the ECM to relearn your driving style.
Throttle Position Switch (TPS)
Digital Volt-Ohm-Meter (VOM)
Jumper Wires (make your own)
Auto Xray Scanner (if available) will eliminate the need for VOM and jumper wires.
Turn on ignition, but don't start the engine.
With a scanner: plug in the scanner and read the TPS voltage. It should be 0.54Volts +/- 0.075Volts
With VOM and jumper wires: disconnect the connector from the TPS. Using your jumper wires, make a connection allowing some room for the VOM terminals to contact the jumper leads and read the TPS voltage.
If out of spec, loosen the two screws holding the TPS to the throttle body, and slightly rotate the TPS up or down, reading the voltage until it comes into specification. Tighten screws. Using the throttle lever, rotate the throttle to WOT (wide open throttle). The TPS voltage should be over 4.0 volts. Close the throttle again, and then slowly open it to WOT, observing the voltage reading. It should increase progressively and in a linear fashion. If it sticks or jumps or falls off at all while doing this check, that could mean a bad TPS switch and could be a cause of stumbling and driveability problems.
After setting the correct voltage, turn off ignition switch. Remove jumpers/scanner and reconnect the TPS connector as required.
At your altitude you should be running additional advance, I can't give you a specific amount to advance your just going to have to advance of a few degrees of the time Till the engine starts running nicely.
Another thing you should do is get WINALDL and purchase or build the cable so that you can run it on your laptop and see what the Howell kit is doing. Here is the link
With this setup you can see exactly what's going on and eliminate the guessing.