503 is a general error code, it doesn't tell anything about the specific error other than that the site is not functional.
If the mojoPortal log is showing frequent application start and end events being logged it usually means that a memory limit on the application pool has been reached and when that limit is reached it will recycle the application pool. One would expect that kind of problem in a low memory environment and it kills site performance. It is hard to believe that kind of problem could happen if there were really 900mb of memory available to the application pool and with the limit set that high. Usually shared hosting provides 200-250mb and that is usually enough to avoid frequent recycling, though more is always better.
I would not expect that a website that has been deployed for several years to start using a lot more memory all of a sudden. Seems more likely that something about the hosting environment has changed, perhaps over time more and more sites are hosted on it and now it is having a hard time keeping up.
My advice would be to ask Arvixe to move the site to another server.
After that I would also recommend upgrading to the latest version of mojoPortal since you are running a very old version (126.96.36.199 which was released in Feb 2011).
Hope that helps,