Hi Kevin,
Looking at your diagnostic and I see:
1. 8GB of Ram - 12 to 16 IMO is needed.
2. Dual Card set up. Doesn't help in Preview/use of PDR.
3. C drive, you have your programs install here - yes? Insufficient working capacity 50 to 100Gb would be better.
4. Data transfer rates between drives causes delays. I have a SDD C drive and I notice how slow the HDD are to respond in normal use.
5. External sources. USB HDD should only be used for backup storage and not for media sourcing. Some USB connect drives go into an energy save mode and need to be pinged to be kept active. I used to use NoSleepHD, a free software.

Preview:
Content, tracks used, images/video and alterations and what additions like Transitions & FX's you've edited into your video. The bigger the load, the greater the lielihood of a slower computer and Preview struggling to display.

I'd look to up the Ram and that might help. Uncheck Shadow edit files.

You haven't mentioned what you're editing and a screenshot of the Edit Workspace with your project insitu would help.

I've made a suggestion and a few guesses, acting upon what I've written is up to you.

Dafydd

I reinstalled it to my E: drive which more space, but it is HDD.

I usually edit 60fps 720p videos.

I keep my recordings in my external HDD, would that be a problem?

I would keep the app on the c: drive - that is an SSD and MUCH quicker. You should see if you can move some other stuff off the c: drive to give it more operating room.

You don't want your PD App and your video footage to be on the same drive.

Dafydd suggested Memory - how much is left when you are editing?

The Hardware looks great as far as I can tell (an SSD to put your video footage on during editing or producing it would be great but pulling 720p footage from a 7200rpm drive should be fine). Win8.1 Pro x64 / Dual x5670 / 24GB / GTX960 4GB / 240GB SSD + 640GB HDD / PD13 Ultimate

The current E: Drive (HDD) has about 300 GB of space, with my installed PD in it.

My SSD is running low on 15GB though, I'll try to relocates files and see if it helps.

Edit: Just remembered, can't really move stuff from my C: Drive, can't find much to move at least. So lets say my C: Drive is out of option for now.

If you want to know what is using up all your space on c: drive then use one of these programs - they are free.

http://windirstat.info/
http://www.jam-software.com/treesize_free/

Your should put your programs on the SSD (fast drive).
It would be nice to put your video on a fast drive also but like for most of us that is sometimes to much $$ and your HDD should be fast enough.
Win8.1 Pro x64 / Dual x5670 / 24GB / GTX960 4GB / 240GB SSD + 640GB HDD / PD13 Ultimate

Still lagging after putting it in my ssd.
I also found out that my PD 13 was on low priority at some point. I didn't change this, and I had to manually switch it to normal.



Also, I did not change any settings. It just started to act like this recently

Update: occasionally after PD freeze, it gave me this error.

Media Source Error [Error Code3]:

A media file in your project may not fully comply with the file format standard, or your system may lack the required decoding components for this file. Please try again using another media file format.

Okay, so I moved one of the clip to my non-external HDD and it runs smoother and doesn't freeze on every few seconds.

Good!

As I said above " put your video on a fast drive" is important.
Internal drives are usually a lot faster than external drives (SATA connection vs USB).
To figure out how fast each drive really is, run benchmark software to check the speed of each of your drives and pick the best 2 (one for the PD software and a separate one for your video source). Win8.1 Pro x64 / Dual x5670 / 24GB / GTX960 4GB / 240GB SSD + 640GB HDD / PD13 Ultimate