Rough run at hot startup

I have a 96 carivan with the 3.0 in it, 80K miles. After it has been run
for a while, then it shut off for a while and restarted - it will run very
rough and has no power for the first 30 seconds or so then smooths out and
runs fine.

Now I don't know a ton about this stuff, but - I have played with a
diagnosis tool on it and what seems funny to me is when I start it the
voltage on the front o2 sensor reads about 1.24 volts. I thought it was not
supposed to be over 1 volt. After it smooths out it reads a sine wave like
I was told to expect, but never over 1 volt.

It also does not do this cold - I guess because the o2 sensor heater is on
and the computer does not read the o2 sensor til it is hot (guessing
here)???

Best I can tell it is the O2 sensor and I should replace it, but I dont want
to if that is not the problem.

Thoughs??? Is there anything else I should look at first??