Sometimes, my car really makes it difficult for me to get good fuel economy.

Normally, this is how my car responds. I can be cruising along at ~50 mpg. By slightly letting off the accelerator, I can get the fuel economy to increase to a variety of ranges between 50 and 100 mpg. If I let off some more, the ICE shuts off.

Here's the problem. On a few occasions, I have not been able to hit any of those ranges between 50 and 100 mpg. I will be cruising at ~50 mpg. By slowly letting off the accelerator, the FE does not increase until the ICE shuts off. This behavior has happened on my most common route that I drive so the problem is very obvious. It does not go away for the entire commute. Each time this has happened, the problem went away the next time I drove the car. Also, the car has been fully warmed up each time this problem has occurred.

Has anybody else experienced a problem like this? Does anybody know what could be causing this?

Thanks.