Win 8.1 RAM full usage without apparent programs using it

Hi,

i have 16GB of RAM (2x8GB@2400MHz) but when i left my PC over night on without any task, in morning it has message that my RAM is full and i have to shut down programs to not loose data. But In Task Manager i see programs like Firefox using at max 500MB, the overall usage of my account on PC is about 1200MB out of 16GB. But the graph is showing usage 15,4GB / 16GB. There is no program visible in Task Manager showing that kind of RAM usage. I have Samsung 640 SSD driver (120GB). Is it problem with that or how can i solve it? The RAM builds up over time if i do anything but its not cleaning herself, for example after 4 hours of using PC:

Task Manager:
In Use 7.0 GB
Available 8.8GB
Comitted 7,3/18,8 GB
Cached 4.6 GB
Paged Pool 405MB
Non-paged pool 4,9GB
Speed 2400MHz
Used slots 2 of 4
Form factor DIMM
Hardware reserved 114MB

It builds up and than at about 15,5GB it tells me i have to shut down programs. It doesnt matter what programs i use, it doesnt seem to be specific memory leak from 1 program.

Sometimes background tasks of "Metro Apps" takes a lot of resources if they are not controlled properly... that's the only thing I could think of with that issue! :/
I think you must use a Memory Optimizer software to help you resolve the issue! Hope that helped!!!

There seem to be similar case here:
http://www.eightforums.com/performance-maintenance/22022-windows-8-memory-management-leaks-5.html

I have Gigabyte G1.sniper Z87 motherboard
GIGABYTE - Motherboard - Socket 1150 - G1.Sniper Z87 (rev. 1.1)

which has Killer E2200 LAN described as possible problem. Also as seen i have the same Task Manager problem - no visible RAM usage of any program yet the graph shows full RAM usage.