Range Rover Classic 1995, 4.2L, V8, 55.000miles: when the engine is warm, at idle, the rpm is very irregular, 900rpm down to 500rpm, up to 700rpm etc. My mechanic tested all cylinder pressure and said one was running on lower pressure than others (thus lower power). In addition there is a clicking noise from the engine which I beleive to be the hydraulic lifters/tappets. Is it worth the trouble/cost to replace those. The noise has not gone worse over the last year. But its just there and annoying. Now the engine is still runnning albeit at some powerloss when warm.