Tire size (23 or 25?)

I agree with the above poster...the tire size is not the problem.  I had some crappy tires that came on my bike (that particular brand sounds familar!).  I think the bike shops deliberately put light/flimsy tires on to make you think your bike is lighter than it is!  Anyway, after a zillion flats I replaced them with Michelin Pro Race tires...23's I think.  L-O-V-E them!  Perfect combo of light yet sturdy.

23 v. 25...Yes, the 25's should have a bit more rolling resistance.  Over the winter to save my Michelins I put heavy-duty armadillo-whatever you want to call them sturdy tires on the bike.  LBS put a 25 up front (again, these were training tires...train heavy, race light!) but the 25 wouldn't fit on the back because there wasn't enough clearance between the tire and the "vertical thing that extends to the seatpost" (heck if I actually know the name).  Anyway, just a thought...if you're thinking of using them for training, get 25's if they fit...it will make you stronger.  If you're racing on these tires, go with 23's.

As for tire pressure...never inflate more than what the tire says!  The poster above said 110-115...which works for MOST tires...but there are some out there that that would be too much for!

I agree - up your pressure and see what happens. 

If you're still having issues and want to buy a new set of itres, the Continental 4000's are a fabulous tire.  I've been riding on them for the last few months and haven't had a flat or a nick in either one.  The only thing I will caution you about is the fact that they're not designed for wet weather... so if you like to ride in the rain, you might want to look at something else.