The first thing to is to change the schedule of the Distribution Cleanup job. By default it runs every 10 minutes which will kill a busy replication system. We run ours once every hour. It sounds like you should also consider a 3 server architecture so that publisher, distributor and subscriber each has its own server.

I don't know how many tables you are publishing or how many publications you have but creating more publications with a dedicated distribution agent per publication could boost throughput.

Hope some of that helps,

Mike