Here is the million dollar question....
 

Just trying to keep my ideas as to what's going on open.

What codes do you have, whats the engine doing?

He has posted the codes several times, he has a bad fuel injector.
Tornado-I dont see how a bad injector would allow the others to have low psi, but I suppose it is possible, but I dont know for sure.
If you fell adventurous enough swap injector #8 with like #4 or something and see if the "bad" cylinder moves to that cylinder.
Just dont want you to drop a million dollars on a new injector if you dont need to.
If it does move then you need a new injector.

Whether stuck open or closed, you'll notice the loss in power. If stuck open, it's gonna dump fuel into your cat and overheat it (and you'd smell the fuel & see the blue smoke). If it's stuck closed, it's going to very likely trip an O2 sensor fault, lean out the mixture, and the motor will bog. Try the test that Spike told ya if you've already identified the potential trouble injector for sure. If you haven't, and you've got an old radiator hose laying around the garage, grab it and put one end next to each injector, and the other end to your ear. If you hear it humming, then the injector is toast. If you hear it making a distinct clicking - then that injector valve is likely fine. If each of the injectors is "clicking" , and you've not noticed a drop in power, then you might get lucky, and it could be as simply as a bent, corroded, or inadvertently dislodged terminal on one of the injector wires (ie, you were doing some work and caught an injector wire and dislodged one of the terminals - took me 3 weeks of banging my head to figure that one out on my Volvo - and it was only by chance that I even noticed it as I was spraying contact cleaner on each injector plug )

I would have to guess yes, if some how an injector is stuck in the open position then it makes sense there would be decreased pressure in the fuel rail. maybe you can remove the injector and bench test it.

X2 on the open one. It could drop pressure. Pop it out and swap it.

Well, after about two hours of cursing and being real pissed off, I got the intake manifold off. It's sitting behind the rear seat now. Talk about a great big pain in the ass. I think it's gonna be even worse to put back together. Ugh....

Anyway, I got the #8 injector out. That was another pain in the ass. What can I do to bench test it?

It is too much of a pain in the ass to put everything back together just to have to tear it apart again. I would rather replace the injector that the computer says is bad and put it back together once.

Here is another question: Will fuel injectors from a DI work in a DII? Might be able to pick some up cheap off ebay.

www.atlanticbritish.com and do some research.
Look up injectors and see what years they will fit.

Yeah, I already looked there. It doesn't appear that they would have fit. The auction is over anyway.

How would I go about bench testing this thing? Could I do it with a couple of AA batteries, or would it be better to do it while it's on the vehicle?I just have to make sure that the other injectors are not plugged in, so I don't get a sudden aerosol of gasoline. that would be...bad to say the least.

Also, I must have not run enough sea foam through that thing, or the way I did it wasn't that effective. The four rear intake ports are clean, almost no sign of deposits. The front four however, look terrible. All kinds of brown crap solidified on the port walls. Would you suggest doing seafoam the right way when I get it back together, or, since the manifold is off, just spraying it with carb cleaner now?

Oh, and the reason I asked the original question was that I had an idea earlier today. If the injector was stuck open, it would just be flooding out the cylinder, causing a misfire under light load. Under a heavier load however, such as when accelerating, or cruising on the highway, the excess fuel being dumped into the cylinder is just being burned, since a heavier load requires more fuel. Hence why if runs like crap in the city, and okay on the highway...does that make sense?