Coolant overflows

Ok guys I need help, but please hear out my situation!! I have a 2008 Volvo D16

I started noticing that my coolant keeps overflowing. More when driving, less at idle. It takes time for it to start leaking though once I turn the truck on. I figured it was the thermostat so I replaced it. The thermostat wasn't opening and closing properly, it was scraping up and down. My coolant overflowing reduced but it's still doing it. So I replaced the whole coolant tank. Still the same problem. I felt I was in denial of a blown hg which might be, but check this out.... I took it to the Volvo dealership, they tested for hydro carbons (exhaust) in the coolant tank, nothing. He pumped it about 30 times and the liquid stayed pure yellow. They did a pressure test, results were perfect. Checked oil for coolant, coolant for oil- nothing.
I'm not over heating, I have no white smoke coming out of my exhausts, I get amazing mileage.

I'm completely at loss for what this could be
Please help me with any advice.
If I missed out to put something, ask I will reply asap. I just don't want to pay thousands of dollars when maybe it's something really simple

 

It over flows to the point where it says "check engine coolant". So I immediately stop and fill it up to the line with 50/50. Also when I replaced the whole tank I got new caps with it. My temp stays normal through out my driving, after going up hill a couple of times it jumps up a little but that's normal. Any other suggestions you might think of because I honestly don't know what to do

 

I swear I think Volvo said 15-17 psi or something, I am not exactly sure..
Ive had this problem even with the old reservoir tho before I replaced it along with new caps. I haven't noticed anything different driving either. It just started one day

 

Could the cabin heater core/copper pipes and clamps cause this type of issue?

Ever since winter, turning the heat on, I notice a sweet smell from the cabin vents. And when my truck sits over night, the copper pipes, at the clamps, leak some coolant as well.

Or is this a whole separate issue that I know I'll have to fix as well.

I was thinking since the water pump has to pull that coolant back to the radiator, it sucks in some air along with it (through the leaky clamp) causing excessive pressure in the coolant tank which is causing the overflow. That's my theory so far. But idk

 

I will try this and see what happens

It just seems that all the coolant that's brought back to the reservoir, goes into the overflow and just drains. It doesn't want to flow back down to where the coolant should be