Q: Car randomly becomes harder to start

I bought my car used last December, so I've had it for almost 10 months. A few weeks after I bought it the check engine light came on, dealership said it was nothing, however it would sometimes be had to start. There was never a time it didn't start but sometimes it would turnover for 5-10 second before it would start. Took it to a mechanic and had the timing valve solenoid replaced in the engine in feb or march and have had no further problems until today. I got in my car, started it and it started fine. Had to run back in the house and when I came back out 5 min later it took about 5-6 sec to start and sounded like it wasn't going to. Battery was replaced in feb/march. What could it be

My car has 108800 miles.
My car has an automatic transmission.