98 Lumina Stalling/Backfiring

I have a 3.1 Lumina/98 and for a while it would do this jerking type motion when I was driving like it just didn't want to 'go'. It was spurratic for a while then became a problem. Then it started having problems at start up. It would turn over and start but it will sputter and backfire and usually it will just stall out. It's loud and a lot of black smoke comes out the exhaust. Attempts at revving it only exacerbates the problem. I usually just have to start it over and over and let it 'work through' it's problem. The weird thing is that if it's really cold outside, the car starts right up and gives me no trouble. The warmer it is outside the harder it is to start. I've also noticed that parking it level or on an incline makes it worse compared to parking it on a decline and it will start right up. I had a diagnostic done on it and was told it was the EGR Valve but everything I've read and everyone I've talked to says that either it's the valve and another issue or just another issue. I'm not sure what to do at this point. That EGR Valve does not come cheap. I read another person was having almost this exact problem but didn't get much help. I am hoping someone sees this and can offer more insight. Thanks :)

The fact that there is black smoke on startup and that it starts more easily in colder temperatures indicates a rich condition. Your experience is not uncommon. An over-rich mixture can be the result of a failing fuel pressure regulator, leaking injector, faulty TPS, incorrect CTS signal, incorrect MAP signal, or significant vacuum leak.

The EGR valve can cause a significant vacuum leak if it is not closing properly.

a faulty ECT (engine coolant temp.) sensor can cause this to happen. they think its colder outside than it really is, therefore overfueling the engine, i also agree with everything else bowtie said, but just for giggles next time it acts up, try tapping on the egr valve with a screwdriver or something, not too heavy though, and also not too hard. see what happens

The coolant sensor sounds like a likely suspect, but it should set a code. I would check the pressure regulator and make sure it isn't leaking into the vacuum hose and sucking gas. When were the plugs changed in this car?

Wow you are from very close to me, small world.