Is that toe measure toe out or toe in? Toe in will be more stable than toe-out. Are you measuring in degrees or inches?
More caster will also track straighter than less. That isn't what I would consider a lot of castor. It is not uncommon for Miata's to run 5 degrees of castor.
Just for grins, have you checked all the ball joints for play, lug nuts for torque, shock bolts for torque, and alignment bolts for torque, control arm bushings for play? While loose lugs usually show up as an off-throttle vibration, they often are not torqued. How about the rear lugs and spring shackles, axle bolts?
Finally, are all four tires at the air pressure you expect them to be?
