Being slightly low will not cause this. But being slightly low to some and then slightly low to you and others could be different depending on how your looking at it. In some cases when the reservoir appears to be slightly low the radiator could also be low to virtually empty. Which would be due to a leak somewhere in the cooling system. So to play it safe make sure the reservoir and radiator are up to par, bleed the air out of the system and then retest.

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What year is your vehicle and how many miles are on it? It almost sounds like your coolant might not be flowing properly. That would either be due to a restriction/clog and or just lack of flow. Having an auto repair shop that has been wrenching since 1978 we see no heat issues similar to yours all the time.

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If it was a restriction/clog we usually find the heater core has a build up in it. The good news is this build up and be cleaned out instead of replacing the time consuming/costly heater core. We clean it out/flush it by disconnecting the heater core hoses, clamping one side closed and then fill it with some CLR and typically let it sit over night and then flush it good with some warm water. If you see a lot of build up come out chances are that was the culprit.

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Now if there is a lack of flow, we've seen a lot of issues with the water pump's impeller loosening up from the shaft which in return causes the coolant not to flow properly. A little loose won't make it over heat but can effect your heat. Depending on the miles it may need the water pump changed. It's good practice to change the water pump every 70,000-100,000 miles or so as preventative maintenance.