2 more 'thoughts' into the pile-
1) I'm with 'cwatters' here - you mention 'high ground' in the take-off direction - standing wave can cause extreme downdraught downwind of such. I think you turned into a wave.
2) If 'lots' of thermals, were there 'lots' of active clouds? Downrush of air from an active cell can ground even a modern jet.
I'm not on the 'over-leaning' nor other engine problem conviction since that would have reduced manifold and rpm, no?
I go for 'weather related' cause/causes