terrible Lag,thats just getting worse by the day

[Greenm]

does anyone know what is wrong?
ive just upgraded my computer to a new one.
at first it was a great improvement in game play,i loved how fast it was running the game.
now the game has become progressively unplayable due to terrible lag.
if i am around 20 of my fellow realm mates,my game is fine.
the problem occours as soon as one enemy player comes on screen.my game slows down terribly.if there are too many of them,it gets so bad im unable to move or turn.
the result of this is,break apart that should only take 1.5 secs to cast,takes aprox 2.5 to 3 secs to cast.winter stroke which is a 0.5 sec cast can take me 1.5 secs to cast.we tested it with a stop watch,the times are not accurate but close enough.at any rate the enemy can run far or attack me 2 times in the time it takes me to cast winter stroke.
my global cool down slows down so much as well.it even skips as it turns instead of the smooth clock like action i get when grinding.
when i am in a group of players who get hit with deafening roar,im not free to move for atleast 2 secs after everyone else.
i see everyone else on my screen move off but im still stuck motionless and unable to move.
my ping rates are 450-500,i know others with same ping rates in my country who have no problems like mine.
my PC has proven it can play this game well,i just dont understand why i cant now only 9 days later.
im using win XP pro,and as i said earlier,the game has run perfectly on this machine's set up.
ive reinstalled the game 3 times now,downloaded the new installer as well.
ive had my internet tested,they say all is fine thier and my end,they have suggested getting a port forwarding to improve ping rates,im unsure if that will be worth the cost.if any cost applies to that.
P.S this loss in performance sucks,dying to a warlock's will domain lvl 3 then staff only attacks,before i was able to get up off the ground,just isnt fun at all ..3008 health gone in 6 secs or less,all by a staff ..nasty nasty lock :P