99 park ave studers cold

I scanned a multiple misfire code so I replaced the ing mod,Worked great for two days. Now on a cold start it will shudder and spit and spudder till warm then its like nothing was ever wrong????????????????But no new codes!
I have already changed lower manifold gaskets,upper manifold,New pvc,flushed the death cool out .Like I said when warm it runs like new,But cold it takes around ten min to clear itself out!? Any thoughts??

3800 gen2 1500000

Will your scanner read engine temp? If so, what is the temp reading before a cold start & after the engine has been running for 2-min?

I used my buds matco scanner he is a second shifter so I wont be able to check temps till tomorrow morning.But thanks,Ill get back with you tomorrow.
Its weard it feals like the plugs are foweld out and need to heat up to clear out and run smooth, But no tail pipe smoke.And I have cleaned the plugs.
Would a wire mis till warm then be ok hot??The wires are only 50,000 but I may change them out to.....Again thanks Stan