Auto mechanics -- Honda minivan question

We have a 1997 Honda Odyssey with 113,000 miles on it. We bought it used about 4 years ago and the van has run like a champ since we got it. Lately, however, when the van is cold and is first driven in the morning, when you push down on the accelerator the van seems to hesitate -- like it's not getting enough gas. As the van warms up, it runs fine. Any idea what might be causing this?

We just had the van at our mechanic's and he changed the oil, plugs, adjusted the timing, checked the fuel filter, and basically tuned it up for us. He said that the computer is not giving him a code and he cannot figure out the problem. The last time we had it in the shop we had to have an O2 sensor replaced.

Any help would be appreciated.

Have your mechanic run a simple vaccum test.