boosted i understand what your saying, ok my buddy has a 2002 civic, i replaced thermo and rad and hard wired his fans to MAKE sure they stay on to see if that was an issue i replace the thermo twice and nothing it would still over heat was stumping me because there was no signs of a bad head gasket no oil in teh water and no water in oil or no smoking so i was wtf... after 3 days of trying to figure it out i had began to notice a slight lose in power nothing "noticeable" but it did and then i was noticing a little more and more air bubble burping from cooling system i burped it for 45 mins kept burping it for that long and air still coming i knew it was the head gasket, it was shoving air into the cooling lines, it was doing same thing yours is bottom tube being cold and only over heating in town. now take from what i described you and see if any applys to what your seeing, i am almost 100% sure its your head gasket the other thing that leads me to believe so is his had just under 100k miles AND had his head gasket just replaced not to long ago, now that's why i didn't think it was his head gasket either but it was. guessing the idiot that did it for him before didn't shave the head after it had already over heated from previous gasket blowing these heads are aluminum and warp easily