Truck Repair: Missing Out under load, speed sensor, inj

Question
I currently have a problem w/ a N-14 500 in a 2000 Freightliner.  The problem that i am having is missing out under load (especially when cold).We have had it to a shop they have replaced engine speed sensor and this didn't fix it.  We took it back and they stated #2 injector wasn't working.  They replaced that--guess what this didn't fix it either.  We now have around $750.00 into this repair and still no fix.  Could it be possible that when fixing #2 injector that another one went bad.  The truck has 630,000 miles.  Any test that a do it yourselfer could do with limited tool ability.  Thanks for your help.
Sincerely,
Nathan Wager

Answer
They are a bit hard to find when the missfire under a load.It is more than likely an injector.Usually have to wait until it does it for 3 secs & drive it down the road cutting them out to see which inj solenoid is hanging up.And you have to have a portable device to do it with.