One thing that caught me a plenty of mechanics out, is the hose that runs from the PCV valve, across the iop of the head and into the air intake.

I had a lot of annoying little problems creeping in and no-one could figure out why. Like a slight miss and a lack of power at times.

After months of this, I was cleaning the engine down and happened to grab that hose and felt a hole underneath, so I pulled it off and half the hose was gone!

From the top it looked perfectly OK.
Anyway, a new one ($80) and all those little annoyances have gone.