I haven't set it up yet, but I don't think it's over-shifting. The chain will shift into the second biggest cog, and then a turn of the pedals has the chain dropping off the chainring.
Odd. Could be different chain stay lengths causing the chain to now be slightly too tight, could be bb with throwing off the chain line. I'd start be redoing the derailleur from scratch and checking the chain length
did you set up the rear derailleur on the new frame? It could just be over shifting.