If the car is truly running that hot in those conditions, you need to tend to it before you overheat it. First step is to get a quality temp gauge (autometer, vdo, etc.) so you actually know what the temp is. General rules of thumb - if it overheats in traffic, but cools off as your speed increases - it's usually not moving enough air across the radiator (fan, shroud, etc.). If it overheats as you speed up, or 'use' the car hard, it's usually an issue of the radiator not having enough capacity to cool (old-clogged, mods require bigger radiator, etc.). If it overheats all the time, the t'stat may not be opening or it could be a radiator capacity issue.
Putting a cooler t'stat in is no cure for an overheating issue. It may buy you a bit of time, but it's not the answer to the problem. It merely treats the symptom, rather than treating the illness. You should have enough radiator capacity and fan capacity to keep the engine running at 1) the t'stat temp or 2) the fan temperature switch. If you've got a 180 tstat, and a fan that's supposed to be fully engaged (clutched mechanical) or on (electric) by 195 -- your temps shouldn't exceed that. In traffic when the fan is moving air, you shouldn't go above the fan temp. And once you start moving so that even more air is crossing the radiator, it should come down to the t'stat temp.
If your 88 has the stock/original radiator in it, it's likely already seen it's best days. I'd have that checked out first.
