Hours-long, uncomfortable gas movements

So I've been having, over the past 6 months or so, on-and-off periods of very uncomfortable gas movements in my intestines.  I can't seem to link this with any sort of foods; it always comes in the afternoon, some time between 1 and 9 PM, and lasts a couple of hours.  I feel gas moving through my intestines, and it's very uncomfortable, but it doesn't seem to actually get out, at least not very quickly.  

I've tried using Simethicone, but that doesn't seem to help much, or at all.  I have no known allergies, and there are none in my family.  It's extremely distracting, and since I'm a hypochondriac, it's giving me all sorts of anxieties and terrors.  I'm 20 years old, male, and in an acceptable state of health.  Does anyone know what could be causing this, and what I can do about it?  I would have thought that if it was something simple like swallowing too much air, that the gas would be in and out rather quickly.  If anyone can offer any advice, that'd be great.

[Lytyr]

Also, I should add that I seem to have a hard time passing gas.  It's like it's dying to get out, but for some reason just won't.

[patient915]

Have you tried eliminating all lactose to see if that takes away the gas?  If not, that would be a good ting to try.