Hi JS,
This is an expected and normal problem if the VCLKs are not locked together. Even the smallest variation in clock frequency will eventually have a problem.
You really only have 2 options:
1) Make your decode VCLK programmable and then slow it down or speed it up based on your buffer state
2) Double-buffer your tiles and repeat / throw away a field or frame if you are going to under or overflow the buffer.
Increasing the JCLK would change nothing.
Dave