Sounds a bit like a microburst or shear between strong up & down drafts.
I got had an 'interesting' experience in a developing TS in a Baron once. Cruising at 8000' then there was an express elevator ride to 12,000. Even with power reduced to 15" MAP. Not much turbulence apart from typical Cu related.
Otherwise I'd think icing with its increased Vs etc.
[This message has been edited by Tinstaafl (edited 16 November 2000).]