i have an 87 na lately the temp gauge has been acting up. the gauge will work fine till it gets to the normal operating range but after that when i slow down and take off again from a light it will drop down to the bottom than shoot up and down from the bottom back to the normal operating range and than stay at the normal range until i stop again

on the side of your thermostat housing there will be a round sensor with a spade terminal coming off it and a black plug that slips on.. i think it's black lol
but i would say it has a loose connection... check the wires out and make sure the plug is firm on there... could also be a faulty sensor but i highly doubt that... check the connections first ;)

Are you having any coolant problems?? Overflow bottle filling up?? Adding coolant often???
A coolant temp sending unit will only work if coolant is contacting it. If the coolant gets low in the engine, the gage will not read correctly. Usually the problem will show up as a gage running in the mid -range, then shooting UP as hot coolant contacts the sender. Personal experience, up and down in speed and acceleration causes gage to fluctuate when coolant is too low.

i have a 87t and the gauge has always been screwy since day 2
it does what it wants when it wants . i've replaced everything i can think of and i've had my share of BHG's curently i have killed the BHG issue and the gauge still goes whacky . i've installed another gauge elsewhere . usually run from 160 - 190 degrees . 160 might be a little cool but making a turbo hot is not difficult