Mine (P4) also suffers from the wheelies and nosedives due to the single-axle drive. Just like over-enthusiastic motor cycling! I spent a long time experimenting with weighting in various positions but never reached a satisfactory solution. But there is no doubting the character of the loco, and if wanting to portray a bashed-about example then it, and Mike Edge's later model, are good starting points rather than the Hornby one. That said, the Hornby version is a superb runner and I managed to convert one to P4 relatively easily.
At the time of it's release the Impetus one was a welcome delight and can still an hold it's own against the other versions when well built.