Thanks to all for your imput, but on looking at the graph, 20 mins. into flight the aircraft apparently dived from 20000 to 2500 then recovered back to 20000 , this happened again from 10000 to 2500, recovering back to 10000 . the aircraft was a Dornier on a short 1 hour flight so I never left the cockpit, strange to say the least.
thanks again
Ken