I think the problem is that Xor_Sily is running 7/24, for months. If memory leak happens, GC will get worse and worse. Maybe I should add some auto restart script later and try again.
You can set the max iterations in the client and run it in a bash loop.
What sort of high performance server is this? If it's virtualized rather than true dedicated, it wouldn't surprise me if the exact amount of available CPU varies dramatically from moment-to-moment even if the provider is guaranteeing some number of cores worth of overall performance.
Could be interesting to some time make a tool for measuring the stability of available CPU, based on running rapidly running a series of identical micro-benchmarks and looking at the variations in how long it takes.