Thanks for all the suggestions on things to consider. Here's a bit more info. The spec. give the weight of the plane as 6-7 pounds. As I mentioned above I'm currently using a Magnum 40 2S with a 10/6 prop. This is definitely the bottom on the range but others here have said it should be OK.

The problems seemed to be primarily with pitch, it'd climb wildly and then dive wildly, But my (frantic) impressions were muddied somewhat by wind effects (which were either mild or strong) that I seemed to be causing some ballooning. No pronounced tendencies to tip stall (thank goodness).

The Spacewalker is a well proportioned low-wing semi-aerobatic style airplane with a fat, almost symmetrical airfoil and generous control surfaces (see photos below). It's been described as a fairly tame flyer and even OK as a second plane.

I've checked and rechecked the balance and it just can't be more than a small bit off. So maybe CG isn't the issue. At that only really leave throw and exponential settings. So at this point I'm thinking of dialing in 20% expo on the ailerons, rudder, and elevator and limiting the throws to 5/8" up and down on the elevator, 3/4" L and R on the rudder, and 1/4" up and down on ailerons. This is a bit more than the minimal throws listed in the terribly manual which almost everyone says to ignore but still less than the "aerobatic" settings. And of course this time I'll wait for dead calm weather!

Who knows I might even get motivated to put in the 52!