Initially is sounds like you are having a signal integrity issue between the FPGA and the 7511. On out ADV7842-7511 reference board with have no problems running 1080p60 over 3 inches and through a jumper. So here's what I'd try
1) change the drive strength on the FPGA outputs.
2) Remove the parallel termination at the 7511
3) Can you tweak the phase of the LLC clock in relation to the pixel bus. The 7842 has the ability to phase adjust the pixel clock in relation to the pixel bus data. We had to adjust this to make it work correctly on the reference board.
4) Sparkles on the output image is often due to bad timing between the 7511 input clock and the input data.
5) Put your finger on the LLC clock line, does this effect sparkle or the image? If so it points to a phase issue between the clock and data.
6) also check the LLC trace length versus the data lengths
All these problems are most prevalent at the higher clock rates. If you change it to work at the higher rates then it should work fine at the lower rates. Here's a thread that describes how to select the right phase. Re: ADV7611 video quality issue