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P0300 missfire PLEASE HELP

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This is driving me insane, 2000 Silverado 4.8 vortec has codes for random missfire and apparently all cylinders are missing about the same amount of times according to the Chevy dealer. I noticed a small intake manifold gasket leak (truck would barely run on cold cold starts till it warmed up) so I replaced the intake manifold gaskets and did multiple smoke tests confirming no air leaks... I have checked each individual spark plug for spark, they all do. Had fuel pressure within spec. Removed all injector connectors 1 at a time and they all made a big difference (all are working in pretty sure).. I replaced pcv valve and fuel filter to be safe. I also plugged up the egr port in the manifold to see if I have a leaky egr system still nothing. The codes the truck also gave are map and maf sensor codes, although the codes were pulled before I changed the intake manifold gaskets so I'm assuming the manifold leak may have caused these sensors to throw codes but I am going to swap sensors with another truck tomorrow. Truck does have 240k on it, but I know this problem is fixable although I have nearly lost all hope as of this point. Any advice is much appreciated, thanks

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