My 2004 Honda Civic just started overheating intermittently. The first time it started showing signs of issues was running a couple marks hotter than normal while driving on the highway. Almost at half. I stopped and checked coolant, which was full. Then the temp got to 3/4 while still moving. I stopped and checked oil and coolant. Both were full. It did not overheat again for a couple of days. But when it did, it was at idle. It got to 3/4 again. I turned the heat on and it kept going up. Heat worked. I pulled over and let it cool down. No steam. The overheating is not consistent or reproducible. It has no thermostat, new radiator cap, burped radiator, radiator flushed, cooling fans work, heat always works and it overheats more with air conditioning on. I can see coolant swirling in radiator with engine running, so water pump works. The car does not leak coolant, but it squirts out of the reservoir when it overheats. Any idea what is wrong with it?