Who's familiar with older RSX Triples?
 

I just got done building on a frame using some older parts, about 1999 Shimano RSX triple STI brifters, triple front derailleur and triple crank. Here's the problem. I am able to shift higher from smallest to middle to largest ring just fine but when I'm in the largest chainring and I click once to go back down to my middle ring, it won't stop at the middle chain ring but it always takes me all the way down into the smallest chainring. I've played with the cable tension but can't seem to get it right. Suggestions anyone other than to part with some cash and buy different brifter and/or front derailleur?

It should work OK, I don't think you need newer parts.
My suggestion is that you go through all of the adjustment instruction steps in sequence found at http://www.parktool.com.
One possibility is that the shifter was not in its lowest position when the cable was attached. The small shift lever needs to be flipped a several times before removing all slack and connecting the derailleur.
Another possibility is that there is too much tension in the cable.

Al

You just need to be patient with adjusting the front derailer. Set your limit screws with the cable tension off and the shifter in the low position. You may need to set, with no cable tension, the barrel adjuster about halfway in (or out) to get the proper middle ring adjustment.

I could demonstrate this way better then explaining it.

My RSX triple works fine. Something's funky - sounds like you need a new(er) brifter.

Check the downtube cable stop. I had a similar problem and found that stop was loose.

Like dan said. go slow and check everything. make sure you aren't off a click. If your shifters are actually on the middle click when the chain is on the big chainring, one click will drop it all the way to the der stop. real easy mistake to make on the fd. try releasing the cable. check everything. clean and lube the brifter. (essentially wash it out with wd40), then re-connect the fd and adjust. sometimes doing it again, you will undo a minor error.

FYI: there is NOT a trim feature on this shifter.

+1 to the above. There are many times when I've wanted it, though. Another thing I just thought of is that the brifter could be gunked up enough that it's not catching that middle position on the way down. Perhaps a hose-out with WD40 might help (followed by Triflow).