last summer my D14A2 starts to do that,if you drive slowly(4.00rpm max)everything fine but if you drive high way or sinilar,its coolant returns from radiator never goes back to it.
Change termostate,fan swchtich,radiator cap,but almost the same and with radiator cap out theres the f**** bubles that show a little wleak somewhere,i hpoe is the headgasket.
i will open my engine this week-end,and then i will see whats happen,i think you better think do the same