Pixelated video

I'm having problem with the quality of my movie. The final bluray dvd or image disc looks very pixelated at the last 15 minutes.
I have the SONY Nx5 camcorder and i record at HD 1080/50i (FX).
I use FCP 7.0.3 and i convert the mts files to .mov with the selection ''log and transfer'' AVCHD Plug in (Apple pro ress 422 HQ) and i have 1920x1080 with 25 fps mov files.
The sequence settings are:
Frame size : 1920x1080 HDTV 1080i (16:9)
Pixel aspect ratio : spuare
Field dominance : upper odd (i tasted and the other 2 choises)
Quiktime video settings
Compressor : Aplle ProRess 422 HQ
100% QUALLITY
The other two tabs video Processing and Reder control are checked with 100% resolution and best quallity.
After editing, i export my movie (after rendering) with current settings so i have 80 GB movie (one hour).
So...i want to create a bluray dvd. I'm using toast titanium 11.After encoding i see
pixels at the last 15 minutes.( i didnt have this problem one month earlierI tested the auto encode and custom encode with this settings...
Video format MPEG-4 AVC
Average bit rate 25.5 mbps
Maximum bitrate 25.5 mbps
Motion Estimation best (unchecked the half-pel)
Reenconding always (i tried never and automatic)
Field dominance automatic.
Aspect ratio automatic.
Nothing better....
I also tasted to share my movie '' bluray dvd'' from fcp .I have the same problem.
I also tasted the Compressor (template for bluray dvd) using these settings...
H.264 video and dolby digital audio.
I still have the same problem....
Is my exported video too big (80 GB for 1 hour) and i have problem with encoding to compress in 25GB for bluray dvd ? Because i had the same problem 3 years ago with dv 720x576 resolution and i start to use Compressor (because without compression my video was too big for a 4,7giga dvd).
Could it be the disc (too slow)? in editing i had problem with dropped frames!
I tried to reder ,export and encode in different internal & external disks but with no luck.
(My discs are connected on a USB2.0)
When i check my export movie, before bluray encoding, everything looks ok (i think)
I can't understand exactly because i see the movie interlaced but after encoding i see the problem clearly!!!!
I have tried to change the exporting and encoding settings, but nothing seems to change this.
Also, i have read many forums for two months and i am confused. bitrates,droped frames, delete fcp preferences from library, redering settings etc...
Would anyone have any idea about what's happening here? Any help will be really appreciated.
P.s. I'm sorry for my bad English
I will be grateful for any help you can provide.
I'm working with iMAC OS X VESION 10.8.5 2,8 GHZ INTEL CORE i7.