I suspect that the "wall heating unit" you mentioned is the culprit.

Your RHP is trying to heat the air in your viv. The wall heating unit is moving air through the unit to heat the room. In so doing, it may have the unintended effect of increasing the air flow in your viv. In essence, it's pulling warmer air out of the viv and replacing it with cooler room air. The RHP is trying to keep up, but frequent turnover of the air in your viv makes it difficult/impossible to stabilize. The farther you move from the RHP, the more likely the impact of the increased air flow. Moreover, the impact would be greatest when the wall heating unit actually turns on. That might explain the fluctuations you're seeing.

A simple test would be to turn off the heating unit for a day or two and see if your temperatures stabilize.