OMG damn bike

Rant time not really constructive, just venting.

So I found a bike shop nice enough to get me in during my lunch hour, and adjust the front derailleur on my bike.  It wouldn't shift into the large ring.  They were quice and it was working great on the bike stand.  I get it home, mount it up on the bike trainer and get ready to start a 60 minute training session.  Everything is going great for the first 5 minutes of my 15 minute warmup.  Then all of a sudden the damn thing starts jumping gears and refusing to shift into the large chain ring again.
The only way I can get it into the large ring is to have the back set of gears all the way down on the smallest rear ring, and that isn't a sure thing, because some times it won't shift when it's there either.
This is 2 days in a row that the damn bike has messed up my training schedule.  I guess I have to go do my workout at the gym on one of their bikes tomorrow. 

I talked to the mechanic about changing the derailleur and his opinion was that the triple front rings are finnicky and I have a cheap one at that.  But he wouldn't recommend upgrading it.  He thought i could upgrade it a couple steps and still may not get the results I wanted, because the whole drive train is pretty low quality.  I asked about switching to a 2 ring system, and he said that would cost me 400-600 dollars, well hell the whole bike only cost 600.00

So I'm paying to get quality Tri training, and I have several rides scheduled for each week, and I don't have a bike that will let me shift into the gears that my workouts call for.

 Some quick thoughts:
First off -- doesn't seem like you can really do anything to screw the bike up more than you've already had to deal with so I say go to town with attempted repairs.  If you don't fix it you're no worse for the effort and if you break something, well it wasn't working to well to begin with.
Either slowtwitch or bikeforums often refer to Sheldon Brown's pages.  I've used it a couple of times and will again tomorrow as I put my race wheels back on and have to adjust my brakes and maybe rear derailer (god I hope not).
Here's a link sheldonbrown.com/derailer-adjustment.html
Another thought -- and the mechanic should've checked it, but how's the chain?  I've had my road bike in the shop twice over the last 2 years and both times they changed the chain for me.  If you look around Sheldon's site you'll see something about measuring chain wear.
Sorry you're having these problems -- good luck at becoming a bike mechanic though (always got to find a positive somewhere in there)

I'm with SAxman on this one.  I don't race bikes but I was an LDS missionary for two years and 10-12 speeds were our primary transporatation source.  When you can't afford top-end stuff, you learn to do-it-yourself.  And sometimes the top-end  sources are really only trying to get you to but new.  It is amazing what a slight cable adjustment can do or a (be brave) bend/twist of a chain guide or derailer arm can do. (like Saxman said, if it breaks,..it wasn't working anyways).  Or see about buying a replacement part and installing it yourself.  Sometimes it takes a shot of WD40 into the cable sleeve, if the cables aren't  moving smoothly or consistently.  Cable repalcement might be  a factor too as they do stretch, especially if they are old or low-end to begin with.

Another option would be to check out some bike dealers or shops that sell used or some other riders that do their own or know some places to take theirs.  The lower end used bike shops are where I have found the best mechanics when I fail to fix it myself.

For the record, I have a used trail bike I got really cheap from an RNYer who went high-end.  I changed the tires to street use and had to clean it up and make some tweaks to get it working right so,... i haven't forgotten everything yet,..(er ah, 30 years later)