Car misfires, jumps for 1-2 minutes after first startup of day. Why?

My 2006 Scion Xa (with only 74,000 miles on odometer) has lately been "jumping" when I first start it up for the first time in a day. I had my local mechanic put it on the scope, but he found nothing. He changed 3 of the ignition coils. No change. He changed a small hose from the ignition complex, which was hard and frayed at the ends. It's a little better, but still jumps when you first start it up. The mechanic drove it around, and it didn't miss. It ONLY does that when you first start it up, first...