help blows hot when driving not when stopped

my 88 4runner blows hot air only when above 2200 rpm, below that it is cold 92% of the time. whats with this? i have replaced the thermostat, should i replace or flush the radiator? new heating core? please help with some advice

I'm not sure how your 4runner is, but many cars use vacuum operated doors to blend air. Engine vacuum would be different at cruise versus at idle. If one of those vacuum doors is faulty it might be blending cool air when it shouldn't.

could be an air pocket in the coolant system. Especially if you have a leak that prevents the system from bleeding the air out. (or a blown head gasket)