Thanks for the help guys, here's what I have done so far: 1. Changed to 4x LVT-45 TVC in the center (I have been using the radial thingies with 4-way symmetry and the subassembly loader). This has mainly solved the problem, however I notice the craft still wants to tumble out of control (only ASAS managed to keep it at 45deg). 2. Changed the NERVA to a Rockomax Poodle in an attempt to solve the inverted pendulum problem noted by Vanamonde. No observable change- might just be me and my eyes though. 3.Checked CoM/CoT. Both are aligned. In both cases the prograde vector still wanders, and the thing is almost impossible to orient towards the horizon at 100,000km Ap to start the orbit burn. Should I go with RCS thrusters now? P.S one thing I noticed is that I never had any of these problems with my Mun rocket- and that one had a lander with big fuel tanks and all the shiny bits like crew ejection systems and whatnot, plus an even bigger version of the main lifter. I don't understand this.