Feb 11, 2009 - replaced intake gasket and water pump due to antifreeze leak
Feb 14, 2009 - travel 220 miles round trip - notice motor missing (when going down hill only). Check engine light on
Feb 17, 2009 - #3 cyl missing - replace all plugs, wires, distributor cap and fuel filter
Continues to miss #3 cyl.(only when going down hill). Check engine light on intermitently. Mechanics baffled, problem ongoing for months. Twice replaced #3 plug only to fowl again
Nov 4, 2009 - replace fuel injector #3 cyl. Continues to miss on #3 when going downhill typically @70-80mph steeper grade more noticable.
Nov 11, 2009 - replace all fuel injectors. No change
Was a compression check done on cylinder #3? Is it fouled with gas, or oil? I think you have bad rings that is letting oil blow by the rings into the combustion chamber when the motor is providing engine braking as you go downhill.
fuel - compression is good
When coasting down a hill the fuel to the injector should be shut off. No fuel should be getting to the cylinder at all.
Just guessing now, since new injectors isn’t taking care for this so it must be the fuel management system is telling that injector to feed gas. Can you try a different known good main computer to see if it is your trucks computer? Otherwise you may have a wiring fault, bad connector or bad ground for that injector.
I read of a knew to me cause of a misfire, it happens when ALL the EGR gases are dumped into one cylinder, this can happen when passeges are plugged. Not saying this is the cause for you, but it was interesting.
What did the fouled spark plug look like? Did it look like one of these? http://www.ngksparkplugs.com/tech_support/spark_plugs/faqs/faqread.asp
WHAT are the trouble codes? P0303, etc?