I had similar experience with GQRX on Pi 3B+. Reducing the sampling rate to 1Msps or lower helped somewhat, but it was still not stable.
For HF, I built a homebrew downconverter using a diode ring mixer and 48MHz canned oscillator. Also tried FST3157 KISS mixer, with even better results.
For demodulation without GUI, CSDR utility works great. CPU usage is in single % range, and the temperature stays low.
73, Mike AF7KR