15fps to 60fps video conversion

You should address the underflow before chasing the 'black screen' since they may or may not be the same problem.

Basically, your system (from SDRAM component all the way through to the DVI connector) need to have enough bandwidth and buffering to accommodate your desired pixel rate and latency/overhead (SDRAM refreshes, Avalon-ST packet overhead, etc.)

Clocked Video Output is pretty simple, and your underflow basically boils down to the FIFO running out of data. Should you care to, you can watch it happen with SignalTap by watching the Frame Buffer/Clocked Video FIFO's and the Avalon-MM Slave in the SDRAM. But for starters, your diagram shows data being pushed onto the FIFO at 100MHz, and popped from the FIFO at 133MHz.

In general, I would recommend the following:

- run your entire system at 133MHz (Avalon-ST, SDRAM, Clocked Video Output).

- make sure the Frame Buffer Avalon-MM master port width matches the SDRAM port width.

- increase the Frame Buffer burst target and FIFO depth

- increase the Clocked Video Output embedded FIFO depth

I'm not familiar with the DE2-115, but you used the word "overclocking" when describing the SDRAM @ 133MHz - what component is on the board and what it is it's maximum frequency?