1992 Dodge Ram - Check Engine Light When First Starting...?

[D.j]

Thanks In Advance To Anyone Who Can Help Me Out Here.

I have a 1992 Dodge Ram, B250, 318. For some reason the Check Engine Light goes on when first started on warm days-(70-80F). When this happens, the engine runs a little rough/shaky. After driving and letting the engine completely warm up, I need to pull over, shut the engine off and let her sit for about a minute. Then when I restart everything's fine. However on cold days-(0-60F), everything's fine.
I went ahead and replaced the O2 Sensor and still the same problem. I don't know if this could be coincidence or not, but I also chaged the oil and that seemed to cure the problem for about 2 weeks and now I'm getting the same thing. This also happened the last time I changed the oil...Coincidence?
Anyone had or heard of this problem and what it could be?
Thanks, Take Care.
D.J