HI Howard. You system specs are only slightly better than mine. You may be pushing the sim beyond its capabilities with all that stuff but not sure.
Most distressing. I had to do a reinstall as well, due to a stuttering mess. I failed to follow my tried and true guidelines. Only install one item at a time, test it out be sure that it works as expected before moving onto the next addon.
I have tons of addons as well but not as many as you. And it does take a long time to install items when you have a plethora of them. My best advice is what I posted above, install only one at a time, ensure its working as expected and not negatively impacting the sim.
First up, does a vanilla install of P3DV4 work perfectly no stutters or bad behaviours, because it should be flawless.
I would say this Howard, I also actually install my addons into a dummy folder, I then copy the items to their required locations, then I test out the addon thoroughly and the sim thoroughly before moving to a new item.
My first experience everything was working flawlessly, I got careless, installed a dozen planes at once some not fully compatible yet, this was a big mistake my sim started behaving badly, forcing in the end a reinstall from scratch.
I am now slowly and steadily adding sceneries and planes and ensuring the sim works perfectly before attempting a new addon. I monitor resources with process explorer, TechPowerup GPU-Z.
Also I purchased Process Lasso to manage background processes to give priority to my P3D and XP.
Load times after a cold reboot do take a long time unfortunately, I don't think there is any way around this except not add the addon's. I find if I shut down P3Dv4 and due some tinkering and fire up P3DV4 again (warm reboot) that my load time is considerably shorter.
My current sim is 445 GB not including the FreeMeshX which is another 60 GB. According to my properties I have 600,000 files in 6140 folders, so that is no slouch.
I find that I have to back the sliders off a touch in large urban areas like London and Heathrow. But rural areas I have the far right.
The key is to be sure of what files are being installed where, and if an addon is acting up you can revert to the last known situation where the sim was purring along. I believe that there are plenty of addons that might cause havoc. For me installing JustFlight non compatible aircraft and porting over REX before they were fully compatible were the culprits.