Something´s wrong with your car man. LX´s were designed to run on the cheapest 87 octane you can find..."they´re economy cars." Anyhow, if getting your octane up solves the prob, its because higher octane fuel burns "slower," so to speak. I would check your timing. If when the air temp goes up the sensor misreads and your ECU over advances the timing...you would get the same problems your experiencing. Likewise, sounds like it might be time for a full tune up, and that includes plugs and wires. If your plugs are fouled they could be holding heat. **If your plugs actually look okay when you pull them, I would recommend going to a little colder plug until you can take the scort to a shop to have the ECU and sensors checked. The colder plugs will hold less heat and let you get back to saving your money on fuel.