Change On multiprocessors, use at most 100% of the processors to On multiprocessors, use at most 50% of the processors.
Save changes to the web-site.
Open BOINC Manager->Advanced view->Projects tab->Select Seti@Home->Click Update.
If you use the local preferences, you will need to change this setting in there. BOINC Manager->Advanced view->Advanced->Preferences->Processor usage. Mind!! Just looking in there and going out with OK will save those preferences and BOINC will start to use them immediately, overriding any of the same preferences from the web-site. If you do not want to use those preferences, click Clear or exit with Cancel.
Fighting for the correct use of the apostrophe, together with Weird Al Yankovic