Kevin, "STP" typically refers to a kind of cable with a different characteristic impedance from EIA cat5, which was used by IBM's Token Ring, and is not Ethernet compatible. There is such a thing as shielded cat5, it's expensive, and if improperly installed it will make the noise worse.
One of the reasons why cat5 twisted pair Ethernet is so popular is that it's very forgiving. I would strongly urge you to avoid power as best you can, and where you can't, either be parallel or perpendicular to it, not at funny angles. While being near power isn't great, it often just can't be avoided.
Incidentally, cat6 cable might help your noise rejection, and be a lot cheaper than shielded cat5e.
>The thing is the UTP cabling is run right next to the power lines in the same conduit. I'm thinking there may be EMI from the power lines.
Whoa there! This does not comply with the NEC. Thank you for playing. You just failed your electrical inspection. (well, you would here, where they're pretty strict) You are not permitted to run low voltage and current carrying cables in the same conduit.
Forget about noise on the line - this violates electrical safety.