Some type of valvetrain problem

as much as i love the first series Discovery's they certainly have their share of problems. Especially the 96's from what i hear. i hear they are famous for valve trouble. This is the first year that in PA we've had a really cold winter, figures, also this winter our disco's thermostat decided to stick fully open. If you know landrovers, you know of the massive cooling system they have, well the water temperatur never warmed up enough for the heater air to get warm, really sucks. well there are new problems. seems like for some reaon its running on less than 8 cylinders.. shakes really bad. and when you come to a stop sign the idle will either be clear up at 1500-1700 rpms or as low as 500 rpms. and anytime its under load there is some mechanical contact noise under the hood, sounds like its coming from the valve train. my dad had it at work and one of the other guys at work suggested it might be a rockerarm bad, after listening to it run and driving it. as far as i think, i'm totally stumped, i have no idea what it could be, mainly b/c the valve symptoms aren't consistent, if something was totally messed up in the valve train you think it would be heard all the time not just under heavy load?? oh well what do u guys think?

here are some pics of our 96, i took it to my senior prom.. haha yaahhhhh

http://www.imagestation.com/picture/sraid54/p98189e79f8ffd858a2ade45e1849119f/fc8adcd6.jpg

http://www.imagestation.com/picture/sraid54/p3f8f1f8d2b284934a3b02fc0282bb289/fc8adcb8.jpg

http://www.imagestation.com/picture/sraid54/p857f107cb8ad1c5c6febb6061930f7d9/fc8adc9a.jpg

http://www.imagestation.com/picture/sraid54/p261822cf32b725a27de6ae1a894eeaa8/fc8adacd.jpg

are there misfire codes?
have you replaced the plugs and wires?
if yes to the above questions sticking valves may be the problem.