I am building a model with a number of parts
Each part is made solid first, which is then added to an existing solid part using Outer Shell
Process is repeated adding the next part.
THEN sometimes Entity Info shows Group only, although they stared off as a solid group
Or
Entity info Shows Solid Group then on save, reverts back to group
Can anyone shed some light on this before I am driven completely crazy
Have you tried either TT’s solid inspector or TIG’s SolidSolver to find out just what is wrong with your shells? My first guess is that you are generating very small geometry where the groups meet, and SketchUp is merging some very near vertices creating holes.
To avoid this, scale the relevant Groups up by 10 or 100 before doing the outer shell operations, and then scale the final result back down.
There is a setting in the preferences too which influences the “repairing” (auto correction), this will probably be the reason for your change while saving the file. You shoud follow @slbaumgartner’s advice.