Has the Average Volume of Volcanic Emissions Been Constant Since the Last Major Fluctuation?
If you have access, I would recommend you to read this short chapter of the Encyclopedia of Volcanoes. In summary:


At ocean ridges, studies show a steady rate of oceanic crust production during the Cenozoic. There have been fluctuations before, maybe due to the breakup of Pangea or other major tectonic events. The present volcanic production rate is ~20 km$^3$ per year.
At volcanic arcs, it is harder to constrain because of the poor preservation of volcanic material. Tephra layers found in deep sea drilling cores show some peaks in explosive activity "in the Quaternary (0-5 Ma), the middle Miocene (c. 13-17 Ma), and potentially the Eocene (c. 37-42 Ma)". This is not well explained. There is also a shorter term variability due to deglaciation episodes: mass unloading depressurize magmatic systems. A conservative estimate of the modern output is 0.5 km$^3$ per year.
Finally, Large Igneous Provinces (LIPs) are very episodic (long-term average of one LIP every 10-20 Ma) but they emit a large volume (generally > 1 million km$^3$) in a relatively short period of time (a few million years). LIP emplacement coincide with major mass extinction events.