P0303 problem after a 1000 miles of no problem after new

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!

You should check to make to sure you have spark and compression first, since that is fairly easy to do. If that all checks ok, then you can proceed to checking fuel. The spider unit you've got was new for 1996 and changed from the 1995 and older central port injection spider. You have individually serviceable fuel injectors that are sequentially fired. If you have a proper scanner, and fuel pressure tester, you can try a fuel injector balance test to see if there is a difference on cylinder number 3 (P0303 code is a misfire on cylinder 3). If you can't do that test, you can try to ohm out each injector and see if there is a noticeable resistance difference. You probably have a bad injector, or some restriction heading into the poppet port of that cylinder. I would remove the spider and look for debris blocking the fuel path too. If the injector ends up being bad, it is fairly easy to change.