rear derailleur and cable problems

I'm pretty sure this has been addressed somewhere before, but I can't find the thread to save my life.

I recently got a new frame, slapped the old components on but with new cables/housing and new front derailleur. The shop routed the shifter cables differently than I'm used to, such that they cross under the down tube. Supposedly, they make a more direct line from the shifters down under the bottom bracket or something. Anyway, it doesn't seem wrong to me to do it this way, but I have no idea if this is affecting the cables in a negative way, so I'm throwing that out there.

I had the shop check the chain when it changed components, even though I'd just had it checked a few months before during a tune. I'd done some more racing on it, so I figured I'd check. It is Ultegra but getting really up there in miles (and racing). It looks a bit worn to me, but the last time I measured (before all this), it was fine, and I am really really good about maintenance with a cleaning lube.

I experienced typical rear derailleur cable stretch after a couple weeks. Well, I'd guess you'd say "typical," because I've never really had to make adjustments for stretch before until the cables got so old they needed replaced (basically, the rear cable would almost give out). Since then, I've had to tighten the cable pretty much every 100 miles or so, and if I don't, then I have to deal with finicky shifting into smaller cogs and a feeling like the drivetrain just isn't running smoothly.

I've now had this setup for about 5 1/2 weeks. It's still driving me nuts

Soo... is this cable just crap? Will this stop? Or, if the problem keeps happening, does this mean that I need a new chain (I *think* I read something along those lines on here)?

off the top of my head and before Deb checks in, did you check to make sure the bolt is tight, and the cable isn't just slipping?

FWIW, it took a long time for the rear derailleur cable on my new bike to settle into the length it wanted to be.

OakLeaf took the words out of my mouth. Make sure the cable isn't slipping through it's anchor. The cable shouldn't be changing every 100 miles. Also check every place where the cable enters and exits the housing, so see that the housing is intact, without wire stands poking from it, and the ferrules are not crushing or splitting.

Is this an MTB? I was told that the cable crossed routing works well on MTBs but isn't as recommended on road bikes. Make sure that the cable doesn't contact the frame anywhere because of the crossed routing.

It's a road bike. The guys at this shop say that's how they do it. No frame contact. Only contact is with the other cable at the cross point. The housing is brand new and looks fine, but it has always made a loud rattle (almost a creak) where the ferrule meets the top barrel adjuster if the cable moves just right (but only on that side). The cable shows no signs of fraying. The rear bolt is tight.

Ok, so someone told me that the cable may not have been "pre-streched" (seated properly at the housing ends). I put the chain in the smallest cog, loosened both barrel adjusters, yanked sideways on the exposed inner cable, and I was able to take up a bunch of slack at the pinch that I couldn't before. Then I reset the indexing, and so far it seems much improved on the stand. Let's hope it holds on the road!