I have had the same problem with the heater blowing hot and cold on other cars, It was low coolant. The was caused by a leak in the system over time.
I recently replaced the hoses on my brat and by the alternator bracket is a "T" off that goes to the carb. There is a plastic valve in that line, that I broke and it leaked there. I removed the alt. mount so I could get to the hoses better, and replaced the valve with a junk yard special that I removed carefully. Hope this helps.