2001 Mitsubishi Montero P0300 and P0175: Runs Rough After Warm-Up, Stalls at Stop Signs

I'm working on a 2001 Mitsubishi Montero XLS that has been exhibiting intermittent running issues after warm-up. The vehicle previously showed P0175 (rich mixture bank 2) and P0300 (random/multiple cylinder misfire). It had a known stuck spark plug for about three years, which I recently addressed by carefully removing the old plug and replacing it with new ones along with spark plug wires. Before the replacement, the engine ran smoothly. After installing the new plugs and an O2 sensor in bank 2 (S1), the car initially ran well without any error codes. However, once warmed up—especially during stops at traffic signals—it began to run rough, nearly stalling, and lost significant power. I've already checked all electrical connections, vacuum lines, and performed a carburetor cleaner leak test with no signs of issues. A notable observation: after the engine is turned off and settles, the upper coolant lines appear collapsed. I replaced the radiator cap to rule out pressure issues, but during testing, these lines remain cool or lukewarm—indicating possible thermostat sticking. I suspect a stuck thermostat could cause improper cooling and affect fuel mixture distribution, potentially leading to misfires in one bank of cylinders. However, I'm not certain whether this would specifically impact only one cylinder or the entire engine system. Next steps include inspecting the fuel regulator for leaks (which I've just completed—no issues found), followed by a full inspection of the intake gasket and injector leak-down potential. I may need to perform a full teardown in the coming days. Any expert insights on how to diagnose or resolve this P0300 issue, especially related to warm-up behavior or misfire patterns, would be greatly appreciated.