engine temps going up and down

2008 X15 with MCX 350. Heater core cracked, assume it wasn't winterized properly. When I Installed the heater I put in a check valve so I could by pass the heater just in case and just in case came up. So I by pass it and stop the leak...all of a sudden the engine temps start rising. They would get to 220 hand around and then drop to 120. Replaced the impeller...same thing. Replaced the tstat and now the engine temps stay at 140 if I am running 20 mph or faster and as soon as I slow down and run around 5-10 mph temps go back to 220 and stay between 180-210.

I blew out all the hoses, nothing in them. Cleaned out the trans cooler. I don't see any leaks.

Could it be the low idle Wye fitting that is plumbed into the intake hose? Any thoughts? Thanks

I plumbed my heater hose in with a ball valve in the same place the low idle y would go. I noticed if I keep the valve all the way open my engine temperature on the gauge never goes over 120. If I shut it 1/2 to 3/4 then my gauge shows normal 170 ish. I don't understand why but I run it 3/4 close just so my temps look right.

Do you still have the low idle Y and just run that before it or do you just run the ball valve? I will remove the low idle Y and see if that does anything. Don't understand why it worked fine before the heater core broke???