Lifters ticking and misfiring at high temps

[okalex]

Well, I posted this on *some other* forum but got tired of waiting for my replies to be approved, so I guess I'll try this forum

Anyway, I've been having a problem with my '87 4WD LE where when I drive it at highway speeds for 10 or 15 minutes, the lifters would suddenly become very noisey and the engine would start misfiring a lot and lose power. I would then have to pull over and let it sit for a while to cool down before I could continue on. Since I want to be able to take the van on road trips, this is obviously a problem.

So I recently flushed the engine and put some Rislone in with the new oil, thinking that maybe sludge is the culprit, and this has improved the situation considerably. I can now drive for about an hour or so before the problem crops up, and it goes away if I bring it back down to city speeds for a few minutes. However, the problem is still present and it's annoying to have to stop for 15 minutes every 45 minutes to an hour when on road trips. I'm also worried that this may damage my valve train if I continue driving on it.

I decided to take the valve cover off and have a look at the valve train, though being a *very* novice mechanic, I'm not quite sure what exactly to be looking for I got the valve cover off last night, and it definitely still seems to be quite dirty in there. My next step is to take apart the rocker arm assembly and get the lifters out. Can anyone provide some advice on what I should be looking for to determine if I need to replace the lifters?