It's possible, very likely even, that this is not a RPGMV issue, but a Windows & graphics driver problem. Corruption of MV cache files aside, try updating your Windows copy, particularly get the KB2670838 update if you don't have it, assuming you're running a Win7. I recommend you look for some kind of Windows activator & debloat tool, to get rid of that annoying watermark in the lower right too. It's really easy to do.
Try these step by step solutions from easiest to most annoying.
Close any games and programs you have running. Unless stated otherwise, try running the game after taking each step to see if it worked or not.
1. Paste the following into your start menu search bar %LOCALAPPDATA%. From the Local folder, navigate to \KADOKAWA\RPGMV\User Data\Default. In the Default folder, find the Web Data and Web Data-journal files and delete them.
2. CTRL-X the entire KADOKAWA folder and paste it to Desktop, then run the game. A new folder should be created in its place. If everything runs fine, even if the issues persist, delete the folder you pasted on your desktop. This was just to check cache corruption.
3. Update your GPU driver. Best to reinstall it completely by uninstalling all old components. This might fuck up your desktop icons. For that I recommend using a program like DesktopOK or something similar, to preserve your configuration.
4. If the issue persists, try saving a profile of your current GPU settings in your NVIDIA or AMD control panel, then reset everything to default and try again.
5. And finally, if nothing worked, update your Windows, especially with the aforementioned KB2670838 if you don't have it. You can Google yourself various Windows activator and debloat tools that will help you customize everything the way you want it to be.