Over the past few days I have noticed some chugging/hesitation while accelerating. It wasn't horrible until today when a few times driving at highway speeds it chugged so hard that I thought it was going to stall. It only seems to do this either when first accelerating from a stop, or once I get over 30mph. Checked my codes using the key turn sequence and it flags a code 21 and no other codes. Idling isn't too rough though. Along with this, I've noticed that my engine temps will get reasonably high compared to normal (reading at about 3/4 full on the gauge) then it will drop down to below half suddenly and climb back up, & repeat. The car does heat up quite quickly while parked, especially for how cold its been here lately (~30f). When I shut the car off, I do hear what sounds to be coolant gurgling and rushing out of somewhere, louder than i've ever heard it before. There are no leaks anywhere that I can see on the ground at all, and my coolant levels haven't dropped at all. Is there any way these issues are connected, as they both popped up around the same time over the past 3ish days?