Premiere projects become laggier and laggier as time goes on...

My premiere project was working great - previewing in full resolution (Ultra HD), and I could edit well since I could play everything back.  Literally, after every day of editing, the footage becomes laggier. Now it is so slow that I actually can't even preview anything even at 1/4 resolution. 

I monitor my PC's usage, so I'm fairly certain it's not RAM (32gb).

Is this something with the media cache files? Since I make sure to save a lot as I work? That's the only thing I think could have changed as time went on.  I have plenty of space on my computer's SSD, and I JUST built this computer, so I'm not quite sure what the problem is.  It could be something with the GPU - but again, the video preview was great at the start and slowly declined over a few days as I continued to edit..

Any help would be great, this is extremely frustrating. I think this could be an issue with premiere..

Thanks,

Claudia