severe FPS spikes

[xXioXx]

So I have had this issue somewhat since ive started playing DDO. ive gone through 3 different graphics cards now, one being a 8600 GTS 256mb (on medium settings low res mode) one being a GTS 8800 512mb (tried both high res and low res high and medium settings) and now on my newer but relatively close to the same card is my GTS 250 512mb(same as before ran on both high res and low res on high and medium settings).. for some reason this seems to not be graphic card related as I am able to adjust all different settings higher and lower and barely effect FPS, but I am for some reason Always catching what feels like lag spikes. the in game FPS drops drastically most the time but somtimes the spike will happen (to the point where the fps doesnt change because the screen freezes) and the fps will already be back up by the time the spikes done. It does seem to drop from an average of 40-60 in most areas down to about 25-30 in major condensed areas (ie: center of market) but it seems like almost any specific movement is causing lag spikes and its not really graphic card related as the spikes will seem to happen even when staring at a wall at 70+fps. it can happen from something as simple as pressing a hireling to come to me, or from just pressing swing, it seems to be initiated due to possibly my internet connection? or maybe my processor just lacks the power needed?
my vista score is a 4.7 (processor being the lowest therefore making my score 4.7 )
im running a pentium D dual core 3.0 ghz processor with 2mb l2 cache (not the greatest this is where im assuming my slowness is coming from? but people with less seem to perform smoother.. atleast more consistant..)
i have tried windows xp 64 bit and now am running vista ultimate 64 bit and it seems to be worse on vista then it was in xp
i have 6 gb of ram (4gb [2x2] kingston, 2gb [1x2] stock) which i thought also may be part of the issue having different ram? they are in the proper slots and bios shows as dual interleaved and have passed memtest in the past (somewhat recently)
As i said am running Geforce GTS 250 512mb but even with my much crappier 8600 gts 256mb was still experiencing these same spikes (actually was pushing higher fps with medium settings on low resolution textures and still recieved spikes just not as many)
my graphics driver is newest from nvidia, windows was JUST installed recently so should be no errors there. game has been reinstalled multiple times during this 2 year or so adventure. I suppose I should just go back to low resolution because it had fewer spikes but my computer really should be able to handle it.. i went through alot to make it capable thinking it wouldnt be an issue to atleast run smooth.. especially after the upgrade from 8600 to the 250.. but even dropping all the settings down i still get spikes.. i tried syncing with refresh rate.. set refresh rate to same as monitor with sync on and off.. tried multiple different graphics settings.. but even setting the 250 to worse settings then then 8600 i still experience great FPS with random spikes..
possibly a motherboard issue? i dont know i have a nforce 4 series amd/intel edition SLI x16 motherboard that was one of the first of its kind and pretty outdates aswell as incompatable with most things.. might be causing some sort of issue there.. im really baffled though its kinda driving me nuts.

let me know what you guys think ill be off to bed for now... will be back tomorrow evening to check in.. thanks for your input