1999 Chevy Blazer

My 99 blazer was running fine one day and then the next it started missing when I would give it gas to accelerate. If you take it easy on the accelerator it will run 90 mph. But if you give it gas a little hard it bogs out and back fires, missess, and jerks. When you let off of it then you can get back in it easy and it will run. We have replaced the Distributor cap, rotar button, spark plugs, Air intake sensor, fuel filter, and battery. We bought a fuel pressure gauge and it showed it was okay but it was just checked at an idle and in park giving it gas. We have ordered a computer but it says on the receipt "must be flashed"-do you know what this means? Any other ideas on what could be wrong?
Thank you