2000 Ford Taurus 135k miles, Duratec, 24vdohc after sitting overnight, on start engine idles then kicks down to about 8 - 900 rpm after a few seconds it kicks down further to about 600 and runs rough for about 15 seconds, almost stalling, before catching itself and returning to 900. I have tried just about everything. Had it in the shop. $450 & a fuel pump later. Same problem. I have replaced the IAC valve & the MAF sensor. Same problem. Had it the shop for wires and plugs, tune-up. They claim no problem. Next morning. Same. Been searching for vacuum leak but so far, nada. Any suggestions?? Seems to be a cold idle problem but I am left scratching head. Time to shoot it??