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It may be best to troubleshoot the model without P-Delta until you are confident that behavior is appropriate. If problems persist with geometric nonlinearity, it may be helpful to run, after the P-Delta load case, a modal analysis which uses Stiffness At The End Of This Stage. Modal analysis may then reveal the source and location of instability.
Buckling can also occur in horizontal floor elements such as beams and floors, if the floors take in plane loads (i.e have no diaphragm or semi rigid diaphragm) and have insufficient capacity, due to structural properties or small stiffness modifiers.
Additional suggestions which may resolve negative-stiffness errors include:
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