Cary,
       check your water pump I worked  in a shop and we had an older Honda Civic with same problem. After much pulling of hair and curseing figured out blades on water pump had rotted off and car had to be spinning at 3000 rpm to get enough coolant flowing to cool car because of the rotted away W/P blades. Something to check, and the po might have flushed coolant before trying to solve problem so coolant might look good but been realy bad before.
 Good Luck
Eric P 1984 Westfalia(Green Hornet)

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