1998 Pontiact Grand Am stalls after a few blocks

I have a 1998 Grand Am with 130,000 miles. A few weeks ago it started to lose power at irregular intervals, usually when it is colder. It starts fine, I go a few blocks then I lose power and the check engine light comes on, and sometimes it stalls. To restart it I have to floor the gas then crank it. Once it goes thru this it runs fine and after a few miles the check engine light goes off. It can be fine for a few days or a couple of weeks then comes back. Problem has gotten worse over the last few weeks (as it has gotten colder). We have replaced the coils, housing etc plus with no real change. The code comes back as 'misfire' with no cylinder indication. The mechanics have not been able to trace anything down.