Sorry but I have no idea why you are having that trouble. Lots of others using the same software without issues even at the same host.
"I've bumped up your memory usage from 614400 to 1614400"
that is not clear what units of memory is available bits bytes what?. My understanding is that Arvixe makes 200MB of memory available in shared .net hosting which should be sufficient to prevent out of memory errors. Less than that would tend to result in out of memory errors mainly because of the way memory is allocated and garbage collected by .net framework you need a good amount of overhead available.
Shared hosting can be a hit or miss thing even with a given host things may work great on some servers and not on others. Arvixe is about as good as most budget hosting, but the main reason I link to them is because they have a good affiliate program it isn't like you hear me singing their praises all the time. Over the years there have been a number of people who have had problems hosting there, most of the time they get resolved, but also a number of people who have had good luck hosting there. Even you I think have other sites hosted there without problems, this is not your first site as far as I know.
I would suggest ask them to move you to a different server and see if that resolves the problem, otherwise maybe it is time to try a different host. I have never heard of a single complaint from anyone using i7MEDIA hosting for example but it may not be as cheap as Arvixe.