If leak downs, plugs, rotor caps and leads have not been checked then that's the first port of call! The rotors on these like to come unstuck and wobble on the shaft, also they tend to corrode fast. If the basics have been done and it is passing the checks then you could indeed have a dodgy lifter, you can check lift at the rocker arm itself I cant remember the figure. All it takes is a minute bit of crap stuck in the plunger valve and the lifters go soft and don't provide full lift, effecting cam duration as well.