2009 g37

Last week I was doing a normal drive when the service engine light came on and the car lost power, stuttering or hesitating past about 3,000 rpm or so. Pulled over immediately thinking a cylinder wasn't firing.

OBD2 scanner read P0128 (90% sure). Lots of coolant under car. Heater tube coupler split. Let everything cool. Replaced coupler. Refilled and spent 30min getting air bubbles out. Everything looked good.

Car worked well next morning. That afternoon, another loss of power (less drastic). Service engine light came on only intermittently. Had to drive 1/4 mile to pull over.

Coolant leak again. Looks like I didn't tighten clamp enough. But this time a burning smell (hopefully just coolant!?). Only lost about a quart this time (resevoir dropped a bit, but was full under cap).

Tightened clamps on coupler, but worried if it's safe to drive? Lots of people have the coupler issue, but haven't seen any with the loss of power. Any ideas?

At no point did I get an overheat warning or see the gauge elevated. Don't understand how this is causing hesitation.