P0303 problem after a 1000 miles of no problem after new heads

96 jimmy 4.3 4X4 144k miles, I rebuilt the upper end both heads machined a 1000 miles ago, bought ebay used injector spider was clean and listed as tested good, all new cap, rotor, wires, and plugs. Started and ran good with no codes for about a 1000 miles now throws P0303 every time you drive it a while 20 miles or so. I can't believe the compression has changed it test good after the rebuild and the electric parts are 2 months old hard to believe they are failing so it leaves the injector spider
anyone tell me how to test the injector without having to pay for dealer visit. Any help would be great!