Thermal Paste could help but usually thermal paste issues are not sudden, heat tends to increase slowly as time goes on.
For sudden overheating issues you should first check if your Fans are running at their normal RPM's
second and as mentioned earlier there are some services running on OSX that can cause CPU utilization to be maxed out at 100% this would explain the heat and the lag. (especially if you are using an old application on a newer OSX revision ie: citrix on OSX Lion will cause 100% Cpu power on startup of the service before you even lunch the app).
for this i recommend to run the windows task manager equivalent under OSX > Applications > Utilities > Activity Monitor and look at the CPU % tab to find the offending task.
It is sometimes useful on Intel Macs to do an SMC reset (and Pram)
the procedure depends on your mac model (if the battery can be removed or not)
For the non removable batteries, you need to have your charger plugged into the AC and the Mac
while is mac is off you need to simultaneously press Shift + Control + Option + Power button for a few seconds and release, the mac will stay off, you will then be able to power it normally.
for Pram you should power on the mac and as soon as the grey screen is displayed before the apple logo appears you need to press simultaneously command+option and P+R
for more info on this subject check this page :
http://support.apple.com/kb/HT3964
