Ran diagnostics

Ran diagnostics. OBDII codes P0101 and P0300 returned. Codes continuous. Service Engine light comes on during operation; will go off on its own. Reappears at random, for short intervals.

Truck has had coolant leak for about a year; codes present during time of leak. Takes about 1 gallon of antifreeze every three months. Drove truck to town (13 minute drive) yesterday, everything fine. Drove home, temp gauge begins going up. Continued to ~240 degrees, still rising when I arrived home and turned the engine off. Same time interval, ~13 minutes. Checked coolant overflow-full; checked radiator; bone dry from vantage point. Added almost a full gallon of coolant.

Drove further today - engine temp spiked when stopped at first light, continued to rise. Pulled off, bought three more gallons, added one more to get back home. Added a third after arriving. No signs of any new, or bigger, leaks that would warrant this kind of coolant loss. Water pump looks fine, hoses look fine (both upper and lower), no leaks at clamps or connections. Trouble codes persistent, leaks persistent, coolant loss persistent and MUCH worse now without ANY new signs of leaking. WHAT DO THESE THINGS INDICATE? I'm an average shade tree mechanic and I understand general automotive theory; I've also seen my share of bad coolant leaks. Nothing I've seen the past two days makes sense. Could I have a cracked/blown gasket, or worse, the block? Also, the heater functions erratically. Maybe that might help shed some light.