If running without the thermostat didn't help, then it is either a clogged radiator or the water pump is not working right.
Since you are the proud owner of an '87, you are in luck! The radiator is all metal! This means you can take it to a reputable radiator shop and have it rodded out!
In this process, they take the top and bopttom off the radiator and, after immersing it in a bath of weak acid, "rod out" the passageways with long brush similar to what one would use to clean the barrel of a rifle.
The buildup is caused by someone using tap water (containing calcium in suspension) instead of premixed coolant or coolant mixed with distilled water (no suspended calcium!) when topping off the radiator.
When the "rodding" is complete, the top and bottom are soldered back on and Viola! A like-new radiator for under 100 bucks!
I had this process done to my Samurai radiator when I was installing a 1.6 8-valve. In nearly 6 years I have had no overheating problems!
Naturally, if it does not help (it certainly will not hurt!), you probably have a water pump problem. When replacing the water pump, you have an opportunity to check and replace the timing belt. If it looks old and/or worn, this would be an excellent time to slap a new one on!
Hope that this helps!