Lagging replay anyone???

Hi
I am running vista and FSX SP2/accellerated on a computer with the following specs;
Q6600 quad core,
ASUS nforcer motherboard,
2gb memory,
2 Hard disk drives with vista on one and fsx on the other(each half full),
8800gtx 768mb GPU,
580w PSU,
and have made sure I have installed all the drivers as I built it my self.
When I come round to playing FSX I am fine with no problems flying around the big airports... with over 30fps at london with traffic settings high and textures at medium and the autogen at high. but when It comes to giving me a reply of a landing or a takeoff using the intergrated 'Instant Replay' sytem which comes with FSX it gives me a simular FPS as usual but it lags ( the aircraft shakes backwards and forwards).
However when I use FSX SP1 on my laptop (really bad graphics etc) I have settings on low for everything and getting 10fps just at london and when I come to use Instant replay I am still getting 10fps but it is not lagging and the aircraft moves as fast as the fps.
I am totally confused about this as I have used totally different spec pc's and the 'lesser' has come off better... anyone else getting this??? and Is there a solution to this problem???
thanks
I am running vista and FSX SP2/accellerated on a computer with the following specs;
Q6600 quad core,
ASUS nforcer motherboard,
2gb memory,
2 Hard disk drives with vista on one and fsx on the other(each half full),
8800gtx 768mb GPU,
580w PSU,
and have made sure I have installed all the drivers as I built it my self.
When I come round to playing FSX I am fine with no problems flying around the big airports... with over 30fps at london with traffic settings high and textures at medium and the autogen at high. but when It comes to giving me a reply of a landing or a takeoff using the intergrated 'Instant Replay' sytem which comes with FSX it gives me a simular FPS as usual but it lags ( the aircraft shakes backwards and forwards).
However when I use FSX SP1 on my laptop (really bad graphics etc) I have settings on low for everything and getting 10fps just at london and when I come to use Instant replay I am still getting 10fps but it is not lagging and the aircraft moves as fast as the fps.
I am totally confused about this as I have used totally different spec pc's and the 'lesser' has come off better... anyone else getting this??? and Is there a solution to this problem???
thanks