Ford Repair: ford ranger 3.0 v6 misfire, valve seals, cylinder misfire

Question
i have a 2004 ford ranger 3.0 v6 and it has a miss. when the engine lite comes on and when i have it checked it always says misfire at startup and either 4,5,or6 cylinder misfire.it does it at idle and all gears and speeds. i have had the heads redone and replaced the injectors, all sensors, coil pack, plugs,wires, its getting proper fuel pressure,and all cylinders have over 150 lbs of compression, when i reset the codes the lite stays off for a while but still has the miss, then out of the blue it comes back on. some days i get good gas mileage but other days i get crappy mileage. i know i have done other things but i cant remember cuz im so frustrated with this. i dont know if this is helpful but i have never had any issues with my passenger side head and when i had the heads redone they had to replace most of the valve seals and other things on the drivers side but said it wasnt warped. it doesnt run hot or use oil even though it has 170,000 miles on it. even though it has the miss it still has power. i am completely confused can u help

Answer
i  hesitate to mention any parts that could be causing your problem for fear you put it on and not having it diagnosed-yours is one i don't want to see during a busy time but when i'm slow i like the challenge-sounds like all normal things have been done so yours is going to be fun for somebody-don't rule out dirty injectors even though you have proper pressure it doesn't mean it is being delivered properly through injectors if it happening on startup and other times i would look at coil and/or cam sensor are you having any chirping noises?