So as a back story my car has run extremely solid since purchasing it from a fellow Fchatter over a year ago. I was driving it last weekend on an extremely hot day and started losing power and it wouldn't want to idle, the slow down light came on, and when i let off the gas it just stalled out. I pulled over and let the car rest, after 20 minutes, the car started right back up and ran fine long enough for me to park it somewhere safe. I then started it back up 3 hours later and it was running strong with all the power, about 10 minutes down the road it started doing the same thing again, this time I just pulled over and had someone meet me with my car trailer to bring it home. I ordered a a test pipe to replace the cat, just incase it's clogged, but I can't imagine that causing all the issues, I also ordered a new O2 sensor. What else should I be looking at and testing?
What year/model/version? (Your profile says 1979 which has two cats and no O2 sensor(s)). Your symptom (wouldn't want to idle, the slow down light came on, and when i let off the gas it just stalled out) is indicative of losing one bank of ignition (not a cat problem) -- you should confirm/deny spark on both banks when the problem occurs.
Need to update my profile, it's a 1984 US model. What would cause one bank to shut down in high heat?
Bad DigiPlex, bad coil, bad coil wire connection, something going wrong inside the distributor cap (like arcing between the dist rotor and the dist shaft or oil contamination), flaky TDC flywheel sensor (or connection) for that bank, etc.. Figure out which bank is losing spark, have a look inside the distributor caps (and if nothing obvious), start swapping components to see if the trouble "follows" one component to the other bank when swapped. Good Hunting!
UPDATE! I went to move the car from my trailer to a storage unit and decided to let it run and do some tests. I let it sit and warm up and it was running fine, I pulled the wire from the cap for the front bank and the rear bank was running fine, switched and all four were running. I went for a quick test drive and after 3-4 minutes of driving I lost a bank or just a few cylinders, engine was't overheating when it happened, just at normal running temp. a few seconds later the slow down light came on. I pulled over and let it rest, it started right back up but still running on only four, i put it into storage. What steps should I diagnose first, could it be a matter of it just running rich and fouling the plugs at temp? Could it be as easy as a busted o2 sensor? I was thinking of letting it run, and pull one plug wire at a time to see if it made a difference and if i had spark... is that step one?
No, not likely. All plugs in one bank wouldn't foul together, nor would one bank foul vs the other bank on your model. Not likely. You can just unplug the O2 sensor and run open loop as a test, if desired, but, again, an O2 sensor fault can't affect one bank vs the other. You could try that, but it is a little dangerous as you can get shocked. It's better to use a timing light or a spark tester to confirm/deny spark presence in a particular coil or spark plug wire.
In addition to Steve's great advice, check each of your spark plug extenders for pin tip sized burn marks. I lost a bank intermittently and the culprit was a burnt spark plug extender. Hopefully your issue is that simple.
Since the slow down light is an indicator of the catalytic converter's temperature, you could have a normal engine temperature but a high cat temperature due to unburned fuel. In this case, it sounds like maybe the cat ECU is doing what it's supposed to - warning of a high/dangerous cat temperature.
