The issue is most likely nothing to do with thermostat.Probably coincidental.
The problem is your unit is old and therefore has lost efficiency. A 29 year old system could be compared to a 29 year old car.
Just as an engine, your compressor has valves that wear over the years.The unit is having to work twice as hard today to produce the capacity that it did 29 years ago.
Along with the age comes loss of actual capacity.In other words when new, it might produce a full 3 tons 36000 btus (3 ton example).After all these years capacity is loss due to wear and now produces only say 32,000 btus which requires it to work harder and run longer.
If you have further questions, please let me know.
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