Lincoln PowerMig 256 losing gas intermittently

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I have a Lincoln 256 that seems to lose shielding gas intermittently. I might weld fine for an hour then it will seem to lose at least part of the gas flow. It will make a little porosity and weld poorly. It has done it for sometime and I haven't been able to figure it out. I can always hear gas coming out of the nozzle if I put my ear up to it. I just think it isn't getting enough? There is plenty of gas in the cylinder, the regulator is set correctly and doesn't change when I'm having issues. The tip and nozzle are clean and the gas isn't getting blown away from air movement. I cant hear gas escaping from any fittings or connections but maybe I am not looking in the right places. If I turn the welder off and come back in 5 minutes usually it will be good as new again. Any help would be greatly appreciated
Reply:your gas valve may be closing or chattering.You will need a test voltmeter and connect it to the gas valve wires. wire will have to stay connected to the valve. Then start welding, have some one watch the gas valve voltage on the meter. See if the reading changes when you see the weld go bad.Good valve should read 21 ohms. the voltage applied to is 6 vdc but pulsing at 333hzIf the coil opens as it heats up the voltage will rise to about 35vdc.So if the voltage drops the pcb may be bad. if voltage goes up or doesn't change the valve may be bad. we have seen several welders of different manufacture the the gas valve just won't open after it heat up with use. The last thing to check is the pressure coming from your regulator. If you are welding with co2. they can freeze and stop letting gas flow. also if the low pressure side acts up and lets high pressure gas through. some times the valve will not open.May have to borrow a different regulator from someone to find this.