1994 Ford Explorer low RPM problem

When I'm at a stoplight sometimes I hear this light thump and my rpm's drop to about 500 and the engine begins to pulsate for a couple seconds. A couple seconds after that the check engine light comes on. Sometimes it turns off after 10 seconds and other time it stays on for 10 or 15 minutes.

This also occasionally happens on the freeway after I have reeched freeway speed. I will be driving for a while and hear this light thump then my rpms go from 2000, about where they r supposed to be, and go up to about 2500 rpms like it shifts down to the next lowest gear or something. Once again my check engine light comes on for a while and it goes off soon after.

I have hooked it up to a scanner and so far all I t has told me is that my oxygen sensor is reading lean, and insufficient rpm change during system test.

Please help if u know anything. Thank u so much.