98 Civic runs fine until warmed up then shuts off when not accelerating

The other day my car had been acting weird, oil light kept flickering after I got an oil leak
fixed. I put oil in and CD2 Heavy Duty Sealer and the car started running better than ever
immediately. Smoother. Quieter. No oil light. It was great. Then the next day, yesterday, it
felt normal again, running fine so I was okay. I drove all day then on my way home it just
turns off on the highway. Whenever I stopped accelerating it would turn off. No check
engine light or anything. No buckle or warning. Just dead. I can start it up again
immediately and it'll be okay until I rev the engine then it goes up, comes down, then goes
off.

This morning started it up to see if I would have the same problem, but it stayed on fine,
even after revving the engine a couple times. I decided to take it for a test drive to see if it
would start, surely as the car warmed up to it's ideal temperature my problem started
happening again.

Is it the fuel pump? If so, is it just the fuel pump? If not, any idea what it might be?