1995 Cadillac Concours

If you have adequate fuel pressure(35-45 psi), and this all happened after getting fuel, I would first take a fuel sample from the line. As far as a quality check, smell it. Good fresh gas will have alot of vapors and a strong fuel smell. Check and see that the EGR is not stuck open, if this is a northstar you won't be able to visually tell. I would tap on the base with a prybar, extension, or ratchet handle, something solid. If it is a 4.9, fell up under the brass colored dome, you should be able to feel the diapragm, and also be able to push it up. Low spark output may cause a symptom like this, as well as restricting exhaust. One way to check if the exhaust is restricted, remove the o2 sensor and see if it will start and run. If it seems fine with the exhaust leak before the converter, than the converter may be restricted.

It will run for a couple seconds when the key is turned on, that is normal to boost pressure for startup until a continuous RPM signal is read.