03 Chevy Tahoe, ran a little rough, than check engine light came

Fuel pump

You have to find the reason the check engine light is on. This will aid in diagnostics. Also, are you buying fuel at the same place both times?

You have so sort of wiring issue, check the battery supply to the fuse box under the hood, it may be corroded.

possibly the o2 sensore

The first thing to check is the ignition timing then the spark plug gaps. There are specific settings/size for the above and must be correct for the car to run smoothly.

have the a/c compressor checked, the a/c compressor might be locking up due to wear and bad bearings. one way to test is to try turning the compressor clutch while the ENGINE IS OFF, if the clutch is very hard to turn or won't turn at all by hand then the compressor is bad. the clutch is what turns the pully, the pully should spin freely, the compressor clutch is on the end of the compressor on the pully side.

Flashing check engine light is missfire, fuel pump probably causing multiple tries to start.Having to build fuel preasure,vac leak on one side of intake probably causing rich/lean condition.Also check back preasure to cat ,Sound like you are moving down right track.Mas air flow would probably set lean code bank1 and lean code bank2

Have the fuel pressure at the fuel rail checked. If you just want to throw parts at it: Fuel filter by the tank first. Fuel pressure regulator next. Order of ascending expense and likely hood. Does the fuel pump hum louder lately? The fuel pump may be on it's way out. It's not getting enough fuel to do anything but idle. Why that is takes some investigation. Somebody like Auto Zone may check your fuel pressure. If the fuel pressure is good, bad spark plug wires are the next guess. Lift the hood and run the engine in the dark (take the light bulb under the hood out if it still works to do this) If you see little lightening bolts traveling over the engine you need new plug wires. If the fuel pressure is good, the wires are good, change the spark plugs. Good Luck!

Go to Advance or Auto Zone and have them run an obd scan on it and it will pin point why your light is on and could solve your problem. This is often done for free.

yes this could be contaminated fuel, pull a sample and let it set in a glass jar, if water is present it will separate out quickly, also could be a load of out of date gas, that will smell like paint thinner if u have it, if this shows ok then check your fuel pressure.