Hi,
I believe this is a common 4ac problem as I have the same. Take a look at where the temp is when this happens, if it';s anything like mine it will be when you come to a stop and you are idling, thats when she'll start to warm up and the thermostat will open and the little banger starts to struggle. The only time I have had it to when it doesn't do this is when I have had new plugs, rotor button, points, clean oil and it tuned perfectly. I also have a thermofan conversion and helped but didn't solve it.
Hope this helps.