91 buick lesabre stuttering

my 91 buick lesabre just recently started do this, when its sitting at idle it will run at about 1000, rpm then it will quickly drop to 500 rpm then back up to 1000 it also does this while dringing and acelorating. My engine is a 3800, and just before this started i replace my alternator. and about 2-3 weeks before that i replaced my oil pump because the old one died. Any ideas what could becauseing this and how to go about fixing this

I would replace the O2 sensor, espicially even you notice flux in the RPM. If you did that I would then check and replace the PCV valve.