GB -
Are you using W95, W98, or WMe? If so, that could be your problem. They use a limited, fixed sized resource pool that doesn't get any bigger even with more RAM. I've heard the IB software consumes a lot of resources. If you combine that with what Datek and your datafeed software consume, maybe you're just exhausting that limited resource pool.
Additionally, the W9x/WMe systems use primative memory management and multitasking subsystems which means you're undoubtedly not getting full benefit or use out of your processor and memory.
The W9x/WMe systems were designed for home hobbiests not for professional grade usage (which trading falls into). You might want to consider upgrading to Windows 2000.