Temperature of water in the shower fluctuates - will a cheap thermostatic tap help?
Get the Plumber back. He may have just cleaned, flushed &amp;/or adjusted the Mixer &amp; not replaced it. But, it sounds like you have a Pressure-Balancing valve instead of a Thermostatic valve. So yes, see if the Plumber can replace the current valve with a Thermostatic valve, this would be the long-term permanent solution.

Just an FYI. Not in all but in some apartment buildings, there can be constant water uses to justify your current fluctuations in that short amount of time. Like, refrigerator ice makers, leaking toilets, building humidifiers, specific laundry machine final rises or someone filling a cup. It may be unavoidable or just unlucky timing with your current valve.