Are you using1.9 pre-release versions? nope 1.8.1
Have you recently upgraded to a new minecraft version? only 1.8.1
Have you installed a new texture pack or mod? this doesnt apparently matter, the lag insists with modded minecraft and vanilla sadly.
Are you using any type of other programs e.g malware scanners, antivirus etc: norton running on the side but this hasnt ever been an issue before hand.
Press CTRL+SHIFT+ESC and open task manager. Right click the javaw.exe and 'set priority' to realtime: i will do when i next have the problem.
If you have a 64-bit OS then consider using 64-bit java (remember to set the default run path to the 64-bit version: checking now, will update