OS X will always utilise as much RAM as possible for best performance. Don't worry too much about that. There's likely to be a different issue.
Try checking the HDD SMART Data to ensure it's not corrupted. Anything other than 'passed' means you need a new HDD.
You can download SMART Utility here.
If SMART Utility comes back clean, try verifying the volume in Disk Utility. If there are any issues, you'll need to repair through the recovery partition (please quote this if you need assistance).
If Disk Utility comes back OK (basically, no red writing), then you can try restarting your Mac with the option 'Reopen windows when logging back in'
disabled. This will prevent any background memory leaks/software conflicts from reopening.
If that still doesn't make a positive impact, and none of the above troubleshooting steps have indicated an issue, try:
- Ensuring the OS is fully up-to-date
- Repair Disk Permissions in Disk Utility & restart
- Reset NVRAM/SMC
Speak with you soon.