I agree, however engine normally will not return to good running condition just by filling back to normal line on coolant, It actually should be the opposite. when coolant is low a vast amount cannot now enter cylinders via failed HG meaning it should run better when low however slightly higher temps would occur due to low coolant. intake manifold does seem more likely.

Have you figured out where the coolant is going? I’m wondering if it could be a head gasket or intake manifold gasket leak. That would mean you’re burning the coolant which can often cause misfires.