+1 on go find another GI. It sounds like you are having a Steroid dependency often referred to as refractory.
Sometimes you actually have to increase the prednisone and taper down more slowly. Diareah is a common side effect of tapering too fast or dependency.
As far as the Pentase, there's a large school of thought that states Pentasa is a useless medication. Many Gi's won't even prescribe it anymore.
You really need to get some further diagnostics done, then a good quality GI will come up with a personalized treatment programme.