What you're describing sounds like the lag that many players experience, especially in game files that are multi-generational or have been played for quite awhile. For me, the lag seems to start at about week 8, and it freezes about every 15 sim minutes or so. The best solution I've found is Twallan's Overwatch mod, and I've found that I don't need to use it continually. I recently had an 8-week-old game that I saved just as my sims were going to bed, then I installed Overwatch and let it run until they started waking up in the morning, saved again, removed the mod, and the game then ran very well. I'm guessing the mod helps clean up and reorganize the way the game file uses memory.
Other than that, you might try the other fixes listed here - cleaning your machine, deleting cache files, updating your OS and drivers, de-fragging your hard drive.
