Gidday Pigs
This sounds like one of any number of things to me. Obvious ones are:
1) Browser is running out of memory/objects that it can use (too many tabs/invocations running at once);
2) Operating system is running out of memory/virtual memory; or memory is too fragmented to run the HTML and Java on web pages;
3) OS is running out of stack space - either the IP stack or one of the other various OS stacks;
4) You have a virus or other malware on your system;
5) your virtual memory file and/or system disk/partition are badly fragmented;
6) ADSL/Cable modem/router has a full DNS lookup table or routing table;
etc, etc, etc.
Without some fairly specific information about your computer, it is rather hard to know what specific thing/s may be causing this.
If you can tell me the browser and version; what OS you are running; what physical RAM and virtual memory settings you have; what anti-virus/anti-spyware s/w you have installed; when you did your last defragmentation run, and what s/w you use for this purpose - I may be able to guide you through fixing this. Feel free to send me a pm about this if you don't want to publish this info to the world at large!!
Otherwise, try shutting down your browser (all tabs/invocations) and re-starting it. If this doesn't work, clear your cookie cache and re-boot your computer and see if the problem ceases.
. 
