Minecraft suddenly started lagging?

I have a gaming PC which is about 4 months old, i have decent specs:
Nvidia Geforce GTX 780 graphics card
AMD FX 8350 8 core 4.0 processor
32GB RAM
Windows 7 Professional 64-bit

I use it for gaming and every game i have ever played is capable of reaching my 144 fps with or without 3d enabled (have a 3d monitor) and when i first had minecraft it worked perfectly. I had the SEUS Ultra shaders mod and a x256 realistic resource pack installed with the Pixelmon mod. They worked perfectly until a few days ago where i started to suddenly experience a drop in fps and it now only reaches 15-20 and it is unplayable for me!

I have dedicated 4gb of RAM to Minecraft alone and i don't know what else to do, can someone please help me sort this out i can't stop it from lagging, if i take out the resource pack or the shaders mod one will work on its own but not together like they used to. I really enjoy using these resource and shaders mods and i don't want to lose them for stupid reasons.

Thanks for any help.
Ben

Added (1). Sorry forgot to mention that my Minecraft has to run with 4gb or RAM allocated because my resource pack won't allow anything less.

ben

YES IT DOES LAG

Sanah

First of all, believe it or not, minecraft runs worse with 4gb allocated than with 2gb. This is because your java (especially if its a 64-bit windows) Will try to use all of it, and since it doesn't have to use 4gigabytes, it will fill it up with junk. Wich makes fps drop. My advise is to lower is back to 2gb or 1gb if that's enough with your
current setting. Also, you may want to remove shaders pack completely, switch to normal resource pack, then re-download and re-install shaders mod. I don't know what's the cause but that usually helps for me.

Also, try creating a new world and see if that one lags too. If only one specific world lags, some chunks in that world may be corrupt. (had that myself) and if that's the case, first try Chunkster. (http://forums.bukkit.org/threads/admin-chunkster.8186/)
it will try to reassemble the corrupt chunks, or, if it has gone unrepairably corrupt, delete it so the generator will re-generate it to what it was before anything was modified.

if chunkster didn't work, try
the Minecraft Region Fixer from here:
https://github.com/...win32).zip

You should extract Region Fixer to its own folder, in this case I should use C:\MCRF\
You should now open up a command prompt (press the windows key + r, then type cmd) and type in the following:
C:\MCRFegion-fixer.exe --verbose --delete-corrupted --delete-entities --entity-limit=100 C:\server\world
C:\server\world should be changed to your world location.

If, once it has finished, it displays something like this:

Found 0 corrupted and 1 wrong located chunks of a total of 1192

then a wrongly located chunk has been found. To fix this, go back up in the command window and look for something like this:

Scanned r.-1.-1.mcr <. : 0, w: 1, t: 354>

You are looking for w: 1 - That is how many wrongly located chunks were found in that region file, and the number may be more than 1… "r.-1.-1.mcr" is the name of the file with the issue. To fix this, go into your world folder, then into the region folder, and delete the file mentioned in the previous step (in this case "r.-1.-1.mcr").

that is all advice i can give you.

Jordy