MJ Ascent Guidance & Rocket Design Issues: Any Advice?

[Starseeker]

So, I've been using the "realistic" guidance mode (forgetting the specific name at the moment) in MechJeb's ascent guidance, and it seems to generally work on stock-scale Kerbin. However, as I make larger designs with lower-TWR upper stages (still a good bit above the 0.4 I've seen cited as a lower limit), it tends to behave oddly, often resulting in not being able to reach orbit (80km) without several hundred extra m/s of dV beyond the commonly-used 3400. One common example of this odd behavior is the rocket entering a coast phase, then moments after starting the final burn, recalculating its profile and pitching up or down nearly 90 degrees while the calculated burn time jumps well beyond what it was listed as during the coast phase.

Any possible reasons why this is occurring? I'll post some screenshots of various rocket designs - as well as MechJeb settings - once I'm able to get them; in the meantime, are there any fairly common mistakes w/r/t MJ autopilot, perhaps in terms of its settings/parameters?

[Guest]

I’m not a fan of MJ’s auto ascent for this reason. Try 

although inspired by MJ’s code this actually works consistently (and works fine in 1.11.x).