There are a host of utilities available for free download which allow older games to access more than 2Gb of RAM. Many older programs can only address that much, regardless of how much your system has available, and if you've got other utilities chewing up some of the lower 2Gb, there's that much less available for the game. It helped with a number of games. Podcat (one of the Paradox developers) released a small patch, so look for "Podcat's .exe" (not sure of the exact file name, which I'm pretty sure doesn't have an apostrophe).
HOI3 was a bit flakey out of the box (I originally bought it on CD), but the later patches stabilized the game, and it ran without any problems for several years....until one of the more recent Windows 10 updates. Now I'm getting an occasional lock-up in HOI3, and it crashed and dropped me back to the desktop yesterday. Several other Paradox games (Victoria 2, for example) will now minimize suddenly for no apparent reason, but USUALLY return to full screen and run after a 10-60 second wait. Occasionally, they fail to return to functionality, and clicking the icon in the task bar does nothing, so I have to shut down the system to restart the game. Other than the Windows 10 patches, there have been no changes to driver files or hardware in the recent past.
Since your problem has been ongoing for some time, I suspect that it's a video driver issue or similar cause, rather than something different in Windows.