Can you try to simplify the project, i.e. remove non-standard controls, dependence on other forms etc., while keeping the bug and then upload the project here? (only lfm, pas, lpi and lpr files packed into a single zip)
Are there any non-standard components compiled into your lazarus? Sometimes strange behavior originates in third-party components. Uninstall them one by one and see whether the bug goes away after removing one particular component.
Moreover, turn off you anti-virus program for a test. We've seen here a lot of strange issues caused by the antivirus going crazy by self-made programs.