Windows Task Manager is notorious for being very very bad at recognizing what's actually running and how much it tasks when it comes to memory stuff, ie it doesn't even show that Windows itself takes almost 1gb. But the listing above says MilkyWay is only 15 MB not GB of memory so that shouldn't be the problem. Try changing the cpu cores available to Boinc for the next bath of tasks to only use 50% of the total available cpu cores, you can do this in the Boinc Manager by clicking on Options on the top line above your tasks and the Computing Preferences and then change the top box to 50%. I think your problem could be that you are using every cpu core, both real and HT and the pc is just bogging down because it can't handle all the swapping around of stuff the tasks do in cached memory. Your pc's are hidden so I can't be positive but that's what it seems like to me.

How many multi-cpu tasks are you trying to run at one time? You may need to check the properties of those tasks, Boinc Manager, Tasks then pick a running one and then on the left click Properties. If each task is using ie 10gb of memory and your pc only has 16gb and you are trying to do something else that needs the memory and cpu too you could be running out of memory and the task may not be designed to handle that situation.