Alfa Romeo Repair: Spider overheating, prestone, bad water

Question
I have a 1977 Spider. It sits in storage a lot. When I started it up this year it overheats after 10-15 minutes of city driving. I took it to a gas station to have the system flushed and they said it was drawing coolant slowly? Needs water pump.
I removed the thermostat - no help.
I unhooked the hose from the top of the water pump, ran the car and it seemed to draw the water I poured into the upper hose intake.
I think I replaced the water pump and recored the radiator many years ago.

Is there a way to clear the system or does it sound like a bad water pump or other problem?

Answer
If you have not kept a 50/50 mixture of glycol in the system then what I have to say does not matter.  The water pump probably has a loose impeller--it it worth pulling it and checking it out--when cold it can appear to be good.  All pumps need a restriction at the output to avoid cavitation (which inhibits flow), in an automobile that is one of the functions of the thermostat.  Put in a 180F thermostat and change it every 2 years.  This system needs to be back flushed. Prestone makes a good kit. Do not forget to back flush the heater. After flushing the system refill with fresh
coolant mix.  The orange stuff is good for 5 years. The coolant turns to a corrosive when its inhibitors wear out. Your system may be severly corroded--in which case the flushing will not completely restore the cooling effect.