When wifi proxy configured, ad blocking works.
However, wifi connectivity drops out. Typically 1-2 minutes functional (sometimes literally ~10 seconds; rarely 10 min. or more).
To recover: either turn off momentarily, and turn back on Wifi, or same with going into and out of Airplane mode.
Confirmed no hardware (phone or access point) problems: iPad side-by-side w/o problems; Android booted in "SAFE" mode (press-and-hold "Power Down" in dialog for SafeBoot) works on wifi reliably, consistently > 24 hours.
Individually ticking off various likely candidate applications which may be causing problem, disabled Adblock-Plus (didn't need to un-install), and removed proxy settings:
Result is > 24hrs uninterrupted good (good throughput) connectivity.
Conclusion: Adblock-Plus is causing the WiFi dropouts.