I am working on simulating a system of three frequncies which are about 5Hz, 5kHz, 500kHz. I found it is really hard because it usually cost too much time to get reliable results, and usually can not  converge. Can anyone give me some advices on how to simulate such systems efficiently? especially, advices on simulating strategies.  :-[

Thanks for pancho_hideboo's reminding.I am sorry I have not described the problem clearly.
The systems I am working on are most micro-electromechanical systems composed of mechanical stuctures and analog circuits. The  structures and the circuit components are modeled with Verilog-a HDL in system level, than they can be simulated by the same analog simulator.The systems built up usually are mixed-model systems, including both signal models and conservative models.
Usually, there are three kind of signals in the system, the sensitive signal from out environment(5Hz), drive signal(5kHz), and the carry wave(500kHz), and all of them are simplely taken as sinusoidal. All the sources such as the drive signals are all given directly by ideal models in order to reduce the compution.
My problem is that the frequncise of sensitive signals and the carry waves differ so greatly that it takes too long time to get a steady result(run several periods ), are there any simulating strategies that can simpfy the simulation for such systems in pure circuits simulation field? I hope I can get some tips from your experience.

ps: Usually the equations of the structures part are much more than the circuits part, and there are connected by coupled models, such as electrode plate, and we can modify the simulator engine we used.