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Diagnosing Heater issue. 2004 Accord

2004 Honda Accord EX; 2.4L
Recent work includes: Oil change, rack and pinion replaced, power steering pump replaced + belt, pads and rotors

Symptoms
Heater is blowing cool air, but not all the time. If in the driveway, 100% cool. If on the road it will be cool, until RPM goes up from either driving on the freeway, or when I tested using a low gear to get the RPM up. Basically, when the RPM is up the heater is functioning OK.
Also, I observed that the thermostat gauge on the dash was showing the temp either normal or quite hot, approaching full hot (would sit around 90%).
The timings would be such that the cool air is blowing and the dash reads hot on thermostat, I get the RPM going and after a few moments (~15 sec) the thermostat heads to a normal position and the heater is sending hot air as it should.

Do these symptoms make the cause apparent?
What steps or tests could I take to properly diagnose or narrow down the issue?

Initial Diagnosis from my searching
Water pump?
Digging through information on faulty heaters pointed me to the coolant levels, I checked and coolant wasn’t super low, but could use more. I still have to get some during my lunch hour to top it off. I did have to replace coolant during the warmer months a few times this past year. I was expecting the loss to be coming from a crack in the coolant recovery tank, but now I am worried it may be from the water pump after watching some videos about heaters not working.

Thanks

Never drive a vehicle when it is overheated. 90% is too hot. You do not have heat due to problems with your cooling system. This needs to be repaired immediately. If you have had to add coolant, there is a leak either internally or externally. When you get that repaired and coolant is filled and bleed, your heater will work, if you don’t damage engine by driving while too hot.

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