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I have a '79 4x4 with the stock engine and a Weber the previous owner put in. I've put about 500 miles on the truck around town since I got the truck and the one thing that's been rock solid has been the engine. It's run like a top the whole time. Until a couple of days ago...

The engine runs FINE when cold. Once it has warmed up, it starts sputtering a bit and the oil pressure gauge the PO installed starts going nuts. Prior to the recent problems the oil pressure would read about 75 when cold, and would drop to about 50 once the oil came up to temp and thinned out. Now it still starts at 75, but once it's up to temp it drops to about 30 when I'm on the gas, and drops to next to nothing or bounces around at idle.

Anybody have any suggestions on where to start? My gut says camshaft bearings, but I'm hoping y'all might have a better (less frightening) idea.

The sputtering when warm and the gauge erratic reading points to more like an electrical problem. Oil looks ok? No water in the oil etc?

Yeah, the oil looks fine. no water or anything there. The coolant looks fine, too.

I should also clarify, it's not so much that the gauge is erratic, but it does bounce when it's zeroing out. It's not like it hops back up to full pressure.

If it were electrical, what might I be looking at? Since it runs like crap, we can be certain it's not just the gauge.

These trucks have known electrical problems with age. Fuse box is a good place to start--loose corroded connections cause intermitent problems of all kinds. You might check oil pressure with a different sending unit to be sure, maybe even screw in a mechanical gauge to verify. If the gas and temp gauges become erratic the same time it usually means an electrical problem.
Sputtering can mean several things, points, condensor, even carb and fuel problems. General tune up.
Oil pressure actually dropping when warm usually dosen't happen all at once, it is a gradual drop from wear over time.
Don't rule out a bad oil filter either, might replace it to be sure.

If it is a Fram filter, ditch it ASAP...

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Thanks for the help guys.

I did a full oil change after the problem started just to rule out filters and the like, and to look at the oil and see what's up. Unless two WIX filters in a row are bad, I think we can pretty safely rule that out. (First one looked fine.)

The fuse box was cleaned up recently.
The gas gauge was recently redone, and it's been steady and accurate through all of this.
The temp gauge is dead.

I ran the truck about 7-8 miles yesterday afternoon with a couple of short stops along the way. The oil pressure (according to the gauge) never bottomed out like it did before, but it did still drop significantly more than usual. It did continue to sputter again, this time even a couple of times while still cold. (Though the problem is much more noticeable and pretty much constant when warm.) While it looked normal before, after this trip it looks like I'm losing a significant amount of coolant. It's still clear of oil and other gunk, but there's less of it now.

I guess the next step is to pull the plugs (replaced about 200 mi ago) and see what they look like?