After warm up I have a very bad miss

First,l it is not timing or a cam problem because both of those would make the miss continuous. It might be the converter, but I doubt that too. Take some Ronsonol lighter fluid and some q-tips and clean out the sockets for the distributor wires. Also clean the underside of your distributor cap. The underside of your distributor cap can collect some carbon build up, which when damp and conduct electricity across the cap between the contacts in the cap.
If you clean the carbon dust from under the cap the problem may or may not go away. I really does sound like an electrical problem. The other thing it could be a cure is to have your computer flashed it may be getting the wrong feedback from your new wires if the wires are new original stock. Wires have specific amounts of resistance and aftermarket wiring may be giving the computer the wrong feedback.

Is it when weather damp if so get some dampseal and spray over all leads etc might just be that easy

Check your fuel pressure regulator. It sounds like it is going or gone bad or the vacuum line has a leak, Been there done that. It will be mounted on the left side of the engine about midway towards the top. It will be round depending if it\'s been changed before or not it could be silver metal color or black plastic.. it will be mounted at the fuel rail. It is vacuum operated. Most of the time when these go bad you will experience a loss of power, missing, it will crank up and run bad till it warms up then you can easy throttle it up to speed, Sometimes it will work fine,poor acceleration it will almost sound like it\'s starving for fuel or can\'t upshift. If any of these sound familiar this may be your problem. Easy check is to pull the vacuum line off and if fuel runs out it\'s bad. They are around 50 bucks or so. You can put a fuel pressure gauge on the rail on the passenger side at the port if you have one or maybe you can talk the auto parts store into a loner while you are there. Now here is the catch that might trick you , you may not get fuel out the line like everyone says you will when it is bad but you replaced it and the truck runs a little better. So what the heck is going on? Well remember I told you it was vacuum operated so if you take the air breather off you can see on the back side of the fuel pressure regulator is another rubber line. Someone thought it would be a good idea to put a plastic line and a rubber vacuum line where you can\'t see it very easy. There is an elbow in the rubber line and it can have a crack in it but you may not see it until you take it off and bend it then there it is the thing is cracked and sucking air so even though you have a good regulator now you don\'t have quite enough vacuum for it to work properly. Replace that and drive the truck for a while till it sort of works it\'s old bones back in place. LOl Hope this does the trick for you it can be a sneaky one f you have the leaky vacuum line too but if it\'s just the fuel pressure regulator you can change it out in ten minutes a pair of pliers to hold on to the retainer clip ( Don\'t drop it ) Pull out the old one slide the new one in make sure it is seated good you may have to push a little hard to seat it. Then put the clip on and you are good to go.

Here is a little trick for you, wait until dark, start the vehicle and take it for a ride and when it starts acting up look under the hood see if there is arcing you will need someone in the drivers seat to give it a slight bit of fuel. i have seen many times wires get changed, left hanging ans the jacket gets melted and when the fuel is given the motor tilts slightly and the wire grounds out and arc's on a piece of metal. you can also put some water in a spray bottle with the vehicle running spray around the throttle body just a fine mist is the gasket below the throttle body is leaking it will bog down. for a couple of dollars the gasket can be change usually removing 3 bolts to do the change. You can test vacuum lines the same way.

I've replaced many intake manifold gaskets on the vortec 5.0 and 5.7. The manifold has very low bolt torque and over time intake gaskets sag and cause vacuum leaks,sometimes internally. Its a big job but a known problem on these engines. Poor fuel mileage, rough idle and coolant leaks at the corners of the manifold are common symptoms. If you have a scanner check for misfires on cylinders besides each other ex. 1-3 or 6-8, the short and long fuel trims will be off too.