Engine won't start?

I have a '97 Honda CRV with about 175k miles. I've never had a problem starting the car in the morning. But after driving the car for a little while, if I stop to run in a store or something and come back out, the car won't start. I try a few times, then let it rest, adn still nothing. A friend told me to put the pedal to the floor when I turn the key in the ignition, and that has worked for the most part. Hit or miss though. I replaced the fuel filter and it started right up, but then drove it around the block, parked, and tried to restart it and the same problem happened. Any idea what could be going on? Also, sometimes when the car starts up it is idling extremely high without my giving it any gas. Other times I have to give it lots of gas and it barely sputters at all. I am stuck! Any ideas what could be wrong? Thanks.