Check Engine Light flashing only when getting on the expressway

So I had to replace the intake manifold due to antifreeze leaking into the motor. I replaced all the spark plugs while it was all taken apart. It has been a month since I did the work, I got a dorman intake but put a K&N cold air intake filter and pipe on and when I get on the expressway having it floored the check engine light will come on and flash for a period of time, but once i let off the throttle the light will go away and not come back on. I do feel like there is a lack of power when the light comes on, like if there was a dead plug. It does not come on in city driving. 4.6 2 valve. Any suggestions.

probably what happened to me on my firebird - one of your plug wires is loose and/or flopped onto the exhaust.

All the plugs are tight and I even have new coil packs. Same issue again today had to to the floor and I couldn't get it much over 80 and the ses light was flashing at me then it shut off and never came on the rest of the drive. Feels like a bad vacuum leak the motor screens but no power coming from it.

Scan it for codes.

^THIS. Could be something as simple as a loose connector plug or something along those lines. Could also be an injector that never got fully seated. Too hard to guess without the code(s).

drdoom said:

Scan it for codes.

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ALWAYS first. From your owners manual page 7.

Light is blinking:

Engine misfire is occurring which could damage your catalytic converter. You should drive in a moderate fashion
(avoid heavy acceleration and deceleration) and have your vehicle serviced at the first available opportunity.

Under engine misfire conditions, excessive exhaust temperatures could damage the catalytic converter, the fuel system,
interior floor coverings or other vehicle components, possibly causing a fire.

http://www.fordservicecontent.com/Ford_Content/catalog/owner_guides/98expog1e.pdf

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so it just says random misfire. when i pulled the engine apart only plug 6 and 8 had water on them so i replace them, and the whole side of coil packs. plugs 1-4 were completely dry but should i check them again. the ses light is on now, but only blinks when i am doing 70-80 under hard acceleration. other than that in the city under hard acceleration it does not come on. so could it be another bad plug or a bad coil pack?