2002 focus idle problem

Hi ever since i bought my focus its had a problem with the engine. The revs go up and down between 800-1200 every 3 seconds or so. Ive been told it cud be the CHT TPS or idle control valve. But i dont wanna go spending loads of money on everything. Any body got an idea of whats wrong? thanks

Forgot to add its a 1.6 zetec wid 74 k miles. Also the idle problem only happens when i dont touch the pedal. When i giv it a little rev it stops for a few seconds then judders again.

I suspect a vacuum leak. Check all the hoses from the intake back. Pinch off the vacuum lines at the source of vacuum ie. the intake plenum. If the sound of the engine changes this is where the leak is coming from. just backtrack the line to check for a leak. Also, the PCV valve could be bad. It burns allows the engine to burn crankcase blowby gas. It is only $3 or so and you can replace it yourself. Just find the largest hose on the intake and follow it back to find the PCV valve. If it is good it will rattle when shaken and you will be able to blow into it one way, but not the other. Given that it could be bad you should replace it. It's reccomended every 30k and is the easiest simplest thing to check out first.

I had a problem with mine but it also did it at stop lights . We changed the PCV valve and it fixed it.