Centrals are caused more by PS then EPAP (which is what ramp affects). This is part of the reason I want to see lower PS to see if it reduces centrals and what effect it has on flow limitations.
The reason to turn ramp off is because your EPAP is low so ramp is doing very little other than not recording data. Your breathing is the same whether you use ramp or not, the machine just isn't recording the centrals during ramp time. That is why we dont like ramp because it both delays treatment and also doesnt report what is actually happening. It is only necessary for higher EPAPs.
Part of the reason your flow rates may have looked bad is what was noticed about your IPAP not triggering. The machine a lot of time was acting like a basic CPAP. This is something to keep an eye on, the easiest way is to monitor the mask pressure graph. It should show a wave shape for each breath shown. If you continue to have sections lack of pressure your machine might be faulty as Sleeprider had mentioned earlier.