2001 ford taurus heating issue?

Ok here is the deal I recently purchased a 2001 ford Taurus 3.0 v6 ( not dohc or flex just plan old Vulcan 3.0) cars runs fine not over heating no noises but I have a small concern the cars heater will run fine for the most part but when I turn the heater on the cars engine temp drops and then warms back up which leads to the heater overall temp to fluctuate I have flushed the entire cooling system and replaced with new coolant plus replaced the thermostat with a OEM Replacement plus yes I used a new gasket it is better but not completely what if anything can be done

Its usually a good idea to flush the heater core separately from the rest of the cooling system or at least that's what I do. I remove both hoses from the core at the firewall, spray water through the larger pipe first then do the same thing to the smaller pipe. You want the water to run clear from the larger pipe before you switch over to the smaller pipe. If you see rusty or dirty colored water coming out, that's usually a sign the core is partially clogged or the tubes are corroded with filth. Its also a good idea to check the inside of each heater hose to make sure no type of spring or valve is being used and if you find one make sure its not clogged with old coolant residue. My Mustang had a spring inside of one of its heater hoses to prevent the hose from collapsing and 50% of the hose passage was clogged because of old coolant and dirt getting stuck to that spring. The factory installs a restricter valve in some heater hoses if the core needs less coolant than the hose diameter allows