In Task Manager, under the "Processes" tab, you can click the "Memory" column to sort by memory used (Might need to be clicked twice). That will give you an idea of what is using up a ton of memory.
I doubt it is The Sims 3. As you said, The Sims 3 can only use so much of your RAM, so it wouldn't be using anywhere near 80-90%. But if any program is using that much memory, the performance of other programs will be affected. The Sims 3 will be at the top or near top of the list, but that isn't worrisome if the number you see being used is under ~3500 MB (3.5 GB), which is the upper limit of TS3's memory usage.
So, my first step would be to find the program (or programs) that are contributing to the memory usage and, based on which programs it is, you go from there to find a solution.
By the way, I found this post about the store content issue that might be helpful to you if the problem with store content arises again in the future:
https://www.reddit.com/r/Sims3/comments/o0ryl3/store_content_keeps_uninstalling/
