Help! Micro seaplane porpoising in flight

[foxtare]

Hi everyone! I just maidened a micro seaplane I built over the weekend, and I noticed a couple of bad flight tendencies I want to iron out before I try flying off of water... First off, it likes to tip stall a little bit, which I think I can fix by correcting my airfoil shape a little bit--I wasn't too careful building my wings. Worse, though, the tail starts flapping up and down once it gets to a certain speed. If I apply much rudder at that speed, it tip-stalls and enters an almost unrecoverable spin. So far, my best guess about what's going on is that the bottom of the fuselage has some deep steps, and I'm guessing these create a low-pressure zone back near the tail, sucking the tail down until the angle of attack increases enough that the tail noses the plane back down, re-creating the low-pressure zone, repeating over and over. Any aerodynamicists out there have some sage advice?

Here's a pic of the plane:

 

[PsyBorg]

Just off the top of my head it looks like the motor is pointing level as it sits but would be pretty steep down angle when the wings are level. As you power up the nose dives losing lift and as the tail gets over the top of the main wing that angle is enough to force the tail down until the main wing is once again above it then starts the same process over again.

I am probably off on this so take my thoughts with a grain of salt and wait for more experienced people to chime in. @localfiend would be an excellent person to poke with a stick for answers on this as he did an awesome PBY a while back.

 

[foxtare]

Sounds like good logic to me! I'll try rotating the motor a little bit.