I use multiple applications all the time, but if it's going to affect things, it's generally my router. My computer loses communication channels with the router. Ouch when that happens to put it lightly. As it may also mean a reboot of the computer on occasions. Luckily it hasn't happened for some time. But yes, I agree UKBob, this may contribute toward this scenario. I guess dependent upon computer config, abilities, etc.

I do all types of video rendering, editing, audio equivalent, or having image manipulative or vector programs running. As well as browser, perhaps windows running on Virtual PC simultaneously, burning, & maybe OS 9 running at the same time as OS X & windows, etc.