99 Suburban Goes From PERFECTLY RUNNING to Rough Idle, poor low-rpm acceleration and codes

[Daddysflying]

Hi all, new user here.
My 99 Suburban C1500 has 114,000 miles. A few months ago I had intake manifolds replaced, along with Crank Sensor, plugs and wires, Dist and cap, rotor. Ran fine for a while, then this:
When first started, it runs sweet, perfectly. After twenty minutes or a half-hour, the idle starts running rough and it throw a misfire cylinder 7 code and a fuel sending unit code. The fuel gauge flips from full to empty, and acceleration at low RPM is hesitant. The entire car vibrates some.
This happens every time it is started and then run for a while. After a while like this, the car began to start hard with long cranking.
My mechanic insisted it is not fuel related, but I changed out the fuel filter and it immediately began starting quickly and the overall running improved, but did not erase the problem entirely.
Here's what I'm thinking: I think the fuel tank is contaminated with debris that settles to the bottom of the tank overnight, resulting in cleaner fuel at the initial startup. Then, as the vehicle runs the fuel contaminants are sloshed into the fuel and into the pump and other parts, which results in the engine running decay.
Is this a possibility or am I being foolish?
Fuel pump is at about 3000 miles, and I know they can be faulty from the box, but it is my understanding that they are not intermittently troubling.
Thank you for chiming in.

[j cAT]

well you did not indicate why the intake manifolds had to be changed/replaced . my guess this is the 5.7L engine. these do run over 200k miles with no problems except the distributor @175K.

fuel pump replacing on this will require the tank to be dropped so that would reveal any debris. then on the fuel pump you always replace the filter. when replacing the filter save and cut open to see if it is full of crap.

new fuel pumps are not a troubleshooting formula. new means nothing. new usually means it probably is the issue.

fuel pressures must be checked using a pressure gauge.

DO NOT USE AIRTEX PUMPS EVEN IF FREE !

AC DELCO OR DELPHI are the recommended brands.

[Daddysflying]

Thank you.

I did cut open the filter I just removed and it had SOME debris; I don't recall how old it is.

I was just wondering if the scenario I described is a serious possiblitiy or if it's more likely that something electrical at the harness is heating up and then unable to work effectively.

We're gonna drop the tank this next week and we'll know then. I will let you know how it comes out.

Thanks again.

[j cAT]

pressure check drive with it hooked up. could be he did not solder the wiring connector or even the ground secured improperly at the fuel pump connector if he did have to cut/splice wires could be the problem.

wire connections also need be sealed with a heat shrink material or liquid tape stuff.