Original issue reads more like an air pocket in the cooling system, water pump can not pump air so you get an "air block" until enough pressure builds up to push it thru heater core, this cause the high temp and no heater heat, then lowering temp and heater heat returns.
It now reads like your heater core is partially blocked.
I would check that first.
Drain some coolant out into a bucket to reuse, just need enough out so alot won't come out the heater hoses.
Remove the heater hoses but at the bypass valve/heat controller valve not the firewall
Point one of the hoses down at the ground
Turn on garden hose, low flow, and hold it against the end of the other hose
Watch the flow out of heater core.
Switch heater hoses, i.e. reverse the flow
Watch the flow
You can increase garden hose flow as you see fit but always start low.
If it seems to be partially blocked, blow out water from core with your mouth on a hose, or compressor but low pressure.
Mix up some warm water and CLR, pour it into the core and wait 15-20min, then flush core again, in both directions
Repeat CLR as you see fit.
Why hot and cold?
If flow thru the core is limited the heat it provides is limited to the replacement of that heat from the engine heat.
At idle the flow is less, so blower fan uses it up pretty quick, at higher rpms the flow increases so replaces the heat in the core.
If core flow seems fine check bypass while it is disconnected.
Take pictures or mark hoses because you WILL forget which hose went where, lol.
When refilling a cooling system unhook one of the heater hoses, highest one, this will allow air to escape while filling up.
Last edited by RonD; 01-30-2015 at 11:49 AM.