How odd. I have no such problems on a far lower spec PC than that.
However, emulating the amiga audio is a problem here, which frankly I don't bother doing (I sometimes use OctaMED to render to disk on the PC, but not for realtime audio playback).
MIDI playback is fine here. I use a JIT 68040 emulation with 8MB translation cache, ECS chipset emulation, 32MB fastmem amd P96 display drivers.