caveat: I don't know the ERJ-145 architecture, so this is more guesswork than knowledge:
Could the fail messages you occassional get be due to a combination of low engine settings and external conditions.
If you have a bleed air anti-ice system and you run a system test at idle conditions, there may not be much energy in the bleed flow. That might mean - especially if it's cold, or precipitating - that there's not enough bleed energy to raise the wing temperature sensor(s) to the operating range, which means the system thinks the WAI isn't actually working and posts the Wing A/I Fail message?
Whereas I'd expect you usually have somewhat higher engine power in flight, and don't get that problem?