Here is the DIY I built.

[ljoe1969]

I built this last summer. Been trying to get the bugs out ever since. The gantry rocks while moving. I can get only 20 inches per min with it. It has a foot print of 48" x 36" lot less cutting area. I dont suggest building one like this.

[CNCadmin]

Very nice, good use of bearing..

[JavaDog]

Have you thought about ditching the L-Channel and going with the traditional gas-pipe. Looks like you could do that without too much modification. Might be an easy way to gain some stability and eliminate the gantry rocking problem.

[ljoe1969]

Currently my plans are to make a new CnC that is 11" x 17" cut area. I think I tried to make too big a machine on my first try.

[ynneb]

I also use the v bearing on angle iron method and it works well. Shopbot also uses this method. Can I suggest a few alterations that will help with stability.
The gantry should be driven on both sides by the one stepper and a drive shaft across its length. This will remove the racking problem. You should also increase its stability as shown below.

You mention that you are considering to make a new machine. Can i say that you are so close with this one that to modify it would be far easier than starting again. The only other mod I suggest its to raise the height of the rails and reduce the height of the gantry sides. If you look at the shopbot you will notice this is how they achieve better stability.

EDIT: the rails can be supported along the full length. This will also help with stability too.