I have .21.1 installed with MechJeb2, ISA MapSat Dev Build, ORDA, and NovaPunch 2. The game will be running fine then sometimes when I Revert to Assembly Building the game freezes up and then comes back with an error message stating that the parts can no longer be found. This includes the stock parts. Is anyone else having this issue? The only solution is to fresh install and put the mods back in. It is also worth noting that this seems to happen randomly.