I frustrated!
Finished a timing belt change 2 months ago it drove great for 300 miles including 2 trips up over the mountain that I live on. Drove down to Gas it up (chevron premium)
it stumbled once like a hickup on way down but seemed fine, left gas station and car started running very rough and no power to it, when I got home (1 mile trip) the exhaust pipe was hot enough to set a discarded rag under car on fire and exhaust system smoking badly. Since this I have replaced plugs, wires, distributor caps and rotors without perceptibal change. (had planed on this work anyway and had parts in house) I have pulled passenger side cam cover and the timing mark on the cam drive gear was still where it should be with the crank at TDC, Compression is 160 to 175 across all cylinders so not that either. So I either have a jammed up exhaust or something wrong in the fuel system causing me to burn raw gas in the cats? Right? Anyone want to weigh in on this, before I start pulling the exhaust apart and ordering new cats My mechanic/best friend is just as perplexed though not a Porsche mechanic he had his own Jaguar/foreign auto shop for 20 years and was a mechanic on McClaren F1 team.
what are we missing HELP

Did a quick Google search and this article seems to cover a lot of the possibilities.

https://rennlist.com/how-tos/a/porsche-928-why-is-my-car-losing-power-383830

I'd likely start by pulling the vacuum lines from the fuel pressure dampers and fuel pressure regulator to determine if there is any fuel leaking passed any of the diaphragms, do the vacuum check for testing for a clogged catalytic converter, and look into the O2 sensors.

Be sure to check that you've got spark from both distributors. The 928 S4 has two separate 4 cylinder spark systems controlled by one EZK spark unit. The engine will run with just 4 cylinders firing but the raw fuel from the non-firing cylinders will ignite in the catalytic converter and could cause a car fire.

As mentioned, a vacuum leak could cause a loss in power.

Good luck,
Rich