PO300 Fault in 1996 Chevy Blazer 4x4 with 126K Miles - Heater Core or Engine Misfire?

Hello! We've been detecting a strong engine coolant odor in the passenger cabin, which has led us to suspect a failing heater core. However, we have not observed any visible fluid leaks or a noticeable drop in heater output temperature. The replacement heater core is currently stored in the living room while we wait to begin the repair. Last night, on my way home from work, I noticed a slight loss of engine power when climbing a hill at approximately 50 mph. I dismissed it initially and completed a 100-mile highway drive today without any issues. However, during our evening test drive, the vehicle exhibited intermittent 'bucking' and significant power loss. We pulled over to a gas station to inspect under the hood, but found no obvious problems. After switching to higher-grade fuel, the check engine light activated. The vehicle ran erratically but managed fine at idle or with minimal load. It struggled noticeably when climbing a hill at 20 mph. Our OBD2 scanner revealed only a PO300 code (Random/Multiple Cylinder Misfire). Approximately 5,000 miles ago, we replaced the spark plugs (AC/Delco), distributor cap, rotor, ignition wires, and the distributor cap. A coil wire failure occurred about a year prior and was subsequently repaired. We're now seeking guidance on what to check first—could it be related to the heater core or is there another underlying cause? Any advice for safely tackling the heater core replacement would be greatly appreciated! Thanks in advance.