Is your Coulter a "Blue tube" or a "Red tube"? I ask because there were some construction differences and I'm only familiar with the blue. Doing anything with the diagonal is very difficult because of the cross bar method that Coulter used to mount it. One of the problems I had was that the glue that was used to mount the diagonal to the frame was too thin and too hard. This actually warped the diagonal and took me a while to find. I eventually gave up on the original as I too could never really get it collimated properly. My solution was to take the mirror (one of Coulter's better efforts) and diagonal and build a completely new scope. This enabled me to really do a good collimation and also shed many pounds to some thing much more manageable. Among the new features is the ability to adjust the mirror while looking thru the eyepiece. I used a bicycle spoke spider so that I also had very complete control of the diagonal position. The whole scope can be broken down for easy transport and takes about ten minutes to put together. Collimation also holgs good during breakdown and reassembly and rarely needs a tweak to improve things. The scope also now sits on an equatorial platform so that things stay in view with out needing constant following, even at the highest power used. This is great at star parties as you don't have to constantly get the object in view after each viewer.
In summary, I would advise ditching the old and build a new scope around the optics. This applies to the blue version. I can't speak as to what would be best for the red.