Performance problems on Raspberry Pi?

Recently I observe problems with playback on low-spec Raspberry Pi. I have two units: RPi A+ with HifiBerry DAC+ and first generation RPi B with HifiBerry DAC. I use them only as AirPlay devices (no plugins, no other methods to stream audio). Both of them are affected with the issue.

Somewhere at 2.185 version playback gets choppy, units disconnect at random and it’s basically impossible to listen. It sounds like DAC has a problem with operations - there are buzz sounds just seconds before cut-out. I played with buffer size, mixer type, DAC operation modes. No dice.

Is it connected to unit performance? How to check that?

Also, any links to firmware from 2.050 epoch? I’d love to downgrade to early 2.x version where all worked well (except perfect music sync for multi-room playback).