Dell Precision m4600. Problems capturing HD video with firewire 4-pin

Hello to all, sorry for my english..
I just bought a Dell Precision M4600 as Avid editing mobile workstation.
For shooting I use a Sony HDV camera, with the ability to record HDV and MiniDV tapes.
When with my workstation I capture (via FireWire 400 4-pin) footage shot in MiniDV, everything flows perfectly,
but when I try to capture footage shot in HDV, the video is full of drops and pixel even Avid stops capture for overload drops!
It seems that the firewire workstation is not able to manage a too heavy video stream.
I tried with different device (players and videocamera Sony, Panasonic), I tried with different tapes and different firewire cables, I tried to change hard drives and software, but I think the problem is just the firewire card, or drivers who run it.
Never happened to have my same problem?
Are there different types (buffer, cache, chipset) firewire 400 with different performance?
Thanks.

 

It may help if you post the full specifications on the laptop here

 

Hi Dave, I really hope you can help me.
This is my laptop specifications:

Dell Precision m4600-
Processor : One Intel Core I7-2860QM (2.50GHz, 8MB cache, Quad Core)
Display : 39.6cm (15.6in) Full High Definition (1920x1080) Wide View IPS RGB LED-backlit LCD Panel
WebCam : 2,1 Mega Pixel
Memory : 8GB(2x4GB)1333MHz DDR3 Dual Channel
HD : 128GB SSD
second HD : 500GB Serial ATA (7,200 rpm)
Controller Raid : SATA o SSD dischi rigidi, NON RAID per 2 dischi rigidi
DVD : 8x DVD+/-RW
Battery : principale a 9 celle 97Wh
Graphics Card : NVIDIA Quadro 2000M with 2GB GDDR3
Bluetooth : Dell 375 Bluetooth scheda
Wireless : EMEA Intel Pro Wireless 6300 (802.11 a/b/g/n 3x3) Mini scheda
Operating System : Italian Windows 7 Ultimate (64Bit OS)

Dell has already replaced the firewire card but nothing has changed.
Thanks in advance, Marco.