This has happened several times most recently today on flight from KDTW to KAUS. At 35000 feet and just purring along out of nowhere the aircraft starts a gradual climb to over 1000 feet above the selected altitude then starts a gradual descent to try and acquire the selected once again. It never quite gets back to 35000 then starts another gentle climb, after several of these oscilations while the aircraft was still climbing I selected flight level change and the aircraft returned obediently to 35000. When this all happened I still had 35000 set in the altitude window of the MCP and the FMA indicated VNAV Path, any ideas?

Bad WX data, what are you using?

Opus Weather

What was your speed tape indicating?

My best guess at this point is somewhere in the neighborhood of between 280 to 300 or thereabouts. Sorry for the lack of accuracy.

I've seen this with OPUS where you will have a sudden pressure rise or drop.  If you look at the live weather map you will see a small purple square in the middle of normal pressure on the surface pressure maps.

 

This sounds like what your seeing but I could be wrong.