It's possible the chain's too long, yeah. In most cases, the chain needs to be shortened by the installer to an appropriate length. With road triples, I usually pick the most chain that the rear derailleur can handle in the small-to-small combo and still not hang limp, then confirm that this is sufficient to go around the big-to-big combo successfully (if not, I add enough to make the big-to-big combo). Once that's determined, I pin the chain.
If the old chain had 6k miles on it, your cassette and/or chainrings may not play nicely with the noOb chain. Skipping under forceful pedalling in some or all rear gears, or full-circumference chain slippage on a chainring, is a sign you're coloring outside the lines with parts that have way different wear levels.
Tangentially: the latest "5700" generation of 105 chains are directional. They're also not recommended by Shimano for triple cranks; Shimano suggests the 5600-series or 6600-series chains (non-directional, in other words) for triples. If you do have a 5700 chain, make sure the non-slotted outer plates with the stamped logos are facing outwards.
Last edited by mechBgon; 07-02-11 at 02:19 PM.