My ps3 is running a little slow lately, not on gameplay?

little big planet, when you start the game you get taught how to move the sackboys torso using the six axis via a little video which pops up while you are playing, that one ran perfectly however i got distracted and missed some of it, so i played it from the start menu and it comes up on a bigger screen that way, but the annimation in it was jerky/laggy audio was fine just the video, i have restarted the console and tried again but same problem, also i noticed it in a few of the fmv's on heavy rain, but gameplay i have had no problems with, other then oblivion goty, it runs slow if for example there are 6 characters casting spells on screen at same time, and when loading a new area but i can understand tht one lol i have 60gb fatty on a standard def tv. all the display settings are correct as far as i know. also can clearing my cache help? how do i do it? will it mess up my save games/trophys, also is there anyway to clean any old unused space on the hdd, like in a pc if it starts running slow you can disk cleanup, defrag etc.. but only if it aint gna mess up my save games etc.., sorry for the amount of questions i have asked but one lead to another lol, if anyone has a copy of little big planet and there ps3 on a standard def tv please attempt to play the video on controlling the sackboy wiv sixaxis from the start menu and tell me if you have same problems. ill give 10 points for best answer