I’m just a home mechanic (with a stained copy of Zinn’s book), so my advice isn’t going to be too detailed. But it sounds like your derailleur needs to be adjusted. I had similar issues when I replaced my shifter cable; the new cable would sometimes cause the gear to suddenly switch on me, especially when climbing.
I would try, in this order:
– Tighten the shifter cable at the derailleur
– Adjust the derailleur limit screws
– Fine-tune with the shifter barrel adjuster
Hope that helps!