This could be a result of a few factors, but there are two common things that bite me;
1.) The computer is always calculating your "end of straightaway" speed, and then using as much boost as it thinks you will need to get there. This means that if you consistently take a corner and hit a top speed of 320kph at the end of the following straight, it will continue to get you to 320kph. However, if you hit the corner extremely well one time, and end that straightaway at 325kph, then every time after that, the computer will continue to try to boost me to 325kph, even though I don't nail the corner that well the majority of time. That means, the computer needs to use more boost to get me there and I run out quicker.
2.) The longer the run goes on, the more of a drone I start to become. Complacency sets in; I'm not braking as hard, which means I'm not building up as much boost. Also, I may become sloppy with my lines which goes back to point number one. Maybe I'm willing to back off and "take it easy", coast a little bit, but the computer doesn't understand that concept. The computer is just programmed to do one thing, hit specific top speeds throughout the lap. So it's constantly trying to compensate for my complacency through the corner by using the hybrid engine to pull be back up to where I should be, until, finally, it runs out of allotment and it can't help anymore.
This is a car that you have to drive at 100% all the time, because the computer is assuming you're running at 100% all the time. What you could do is just turn the Trim down for a lap or two. Instead of "Manual,4", try running in "Manual,5-6" for a lap or two to rebuild the allotment back up.
Also, look for places in the lap where the car may be using boost that aren't really worth it. If you hit Indianapolis right, the car shouldn't use any boost between Indy and Arnage, but if you're a little slow through Indy, the computer may use a little boost to try to make up the difference. The same happens in the middle of the Ford Chicane. Usually I don't use any between the two chicanes, but every once in a while, it will use a little squirt. These little squirts add up over time, taking away your allotment but not really contributing to the lap time.
You could counter these sections with the "No Boost" button, but A.) there is no indicator whether "No Boost" is activated or not, so it's up to you to remember to turn it on/off, B.) if you're going to hit a button that much, it may as well be the "Boost Hold" button you're pushing instead, as you always know when you're using it and you'll always be using it on a straight.
