Extensive investigation revealed plastic pipe blank lodged in fuel feed pipe in pylon area. During high thrust setting, ie take off, fuel flow to engine HP pump was blocked, then almost immediately cleared, then blocked again, causing fuel starvation etc and surging. Initial problem was put down to failure of the original engine, as after the replacement was fitted full power runs were carried out. Offending plastic blank was traced back to manufacturer....
Exactly the same thing happened to my old VW Bug, only in that case it was a pencil point that had broken off and left floating around inside the gas tank (previous owner must have used a pencil to plug up the gas outlet!). Car would unexpectedly stall, but only while driving on the freeway. Took a while to track it down, but the fix was easy.