Unable to activate skills even though UI displays them as available for use.

I have stable connection, my ping to Montreal datacenter is 120ms.
video footage: http://youtu.be/IgjjWCqOWmw
Despite UI displaying skills as available for use, in fact they are not activating for some time, some hidden cooldowns not represented in UI are in effect and they are inconsistent throughout different abilities.

This hinders combat experience as important abilities do not go off predictably, it also promotes the habit of continuously spamming the button, because you are never sure that the ability will go off when you press it just once.


Time: 2013 09 10 around 11:30 am GMT 0

Frequency:
Always

World name:
Cerberus

Character name:
Soul Pierce

Class/Level:
Archer(Bard) 50

Party or solo:
Solo

Area and coordinates:
Coerthas Central Highlands->Dragonhead 24X 20Y

Steps:
1. Activate weaponskill "Heavy Shot"
2. Try to active instant off GCD ability "Bloodletter"
3. Bloodletter won't activate for around 1 second despite it not being on GCD nor cooldown.

This can be reproduced with almost any instant-cooldown/cooldown-instant or instant-instant ability combination:

1. Activate ability "Internal Release"
2. Try to active weaponskill "Windbite"
3. Windbite won't activate for around half of second, despite it not being on GCD nor cooldown.

1. Activate ability "Bloodletter"
2. Try to active weaponskill "Straight Shot"
3. Straight shot won't activate for around 1 second, despite it not being on GCD nor cooldown.

1. Activate weaponskill "Heavy Shot"
2. Try to active instant off GCD ability "Blunt Arrow"
3. Blunt Arrow won't activate for around 1.2 seconds, despite it not being on GCD nor cooldown.

1. Activate instant off GCD ability "Barrage"
2. Try to active weaponskill "Heavy Shot"
3. Heavy Shot won't activate for around half of second, despite it not being on GCD nor cooldown.

Connection Specs
- Type of internet connection/provider: TEO
- Modem maker/model number: TP-Link TL-WR1043ND

PC Specs
-=-=-=-=- System Information -=-=-=-=-
Time of this report: 9/10/2013, 12:00:22

Operating System: Windows 8 Enterprise 64-bit (6.2, Build 9200) (9200.win8_gdr.130531-1504)
Language: Lithuanian (Regional Setting: Lithuanian)
System Manufacturer: System manufacturer
System Model: System Product Name
BIOS: BIOS Date: 02/24/10 14:02:48 Ver: 08.00.15
Processor: Intel(R) Core(TM) i7 CPU 920 @ 2.67GHz (8 CPUs), ~2.7GHz
Memory: 6144MB RAM
Available OS Memory: 6136MB RAM
Page File: 5429MB used, 1665MB available
Windows Dir: C:\Windows
DirectX Version: DirectX 11
DX Setup Parameters: Not found
User DPI Setting: Using System DPI
System DPI Setting: 96 DPI (100 percent)
DWM DPI Scaling: Disabled
DxDiag Version: 6.02.9200.16384 64bit Unicode

-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-

Browser Information
- Name and version of the browser currently in-use: IE 10
- Internet Explorer Version: IE 10