If the car itself wasn't used for months, I might think the gasoline has gone "stale"? That's what the people who do auto-LPG kits tell their customers NOT filling their gasoline tank to the full level because if gasoline is not used within a certain time frame (months). (Since auto-lpg converted cars only use gasoline to start and lubricate, a full tank can last a few months). It would go "stale" or change in octane value due to vaporization of the more volatile parts of the mix.

To solve, emptying the gasoline tank of the old gasoline and pumping some fresh hi-octane gasoline might solve the problem. Also using a different brand of hi-octane gasoline might help.