Ah, you hit my pet peeve with BOINC, PET PEEVE actually with BOINC, small/short running projects with aggressive behavior from the get go.

- Let's start with setting the (in poorest English) "Switch between applications between every xxx minutes". The default is 60 minutes. Set that to a lower value than the run times of PrimeGrid. If it's 45 minutes, set it to 30 minutes. You can do they via the My Grid > Device Manager > Device Profiles > Custom options or via the BOINC Manager Advanced View Advanced menu > Processor section, somewhere near the bottom.

- Also, if you have played with the additional buffer / cache setting, set that value to the shortest project deadline. If PrimeGrid is 3 days and your cache is 4 days, you got panic, the client always finishing work and fetching more until it's deeply in debt.

- Same with the connect intervals, it has to be shorter than the shortest deadline. The deadline of 0.1 days should be fine.

Let us know, and we'll take it further from there.

edit: Now I've spoken up first... never saw the word, only heard so it's spelled PEEVE.