FPS dips can be caused by various things, but they are always a bottleneck in the system.
If it happens with the combination of specific effects, and is a steady FPS value, then the effects are probably too heavy for your GPU to handle.
Or the machine could be thermal throttling.
If you see momentary dips, then it could be some background task, or another program eating up system resources for a short time. Disk read bursts, or CPU usage spikes at the same time as the dips, could be a sign of this.