03 Eclipse running hot despite replacing thermostat

I have an 03 Mitsubishi Eclipse 2.4L and I'm having overheating issues. I thought it was the thermostat so I replaced it and found I was still having issues. My car would run fine until I got on the highway and it would go up between 2/3'rds to 3/4'rs on the temp gauge. I recently adjusted my thermostat to have the jiggle valve straight up as per the manual and my car started get almost all the way up to red as soon as I hit the freeway. I don't believe it's a flow issue because when I removed it, it looks like new coolant and old coolant were both coming out of the engine (i forgot to drain it correctly) I'm lost as to what the issue could be. Right now, I have the thermostat completely removed but I don't want to drive it that way. What do you believe I should do to further troubleshoot this and what would be your best advise as to my next course of action?