I've been having intermittent idle stumbling and general idle instability. If you don't know when the vacuum lines were changed or they obviously look original, I suggest you replace all of them first. Just to understand the effect, remove one of the small wiper fluid line sized lines with the engine running and block/unblock with your finger a few times and you'll notice the idle surge a bit. It seems to be less severe now. The amount of vacuum leak can change as materials contracts and expands with temperature. Rubber hoses mitigates this, but only if they're flexible. Many of rubber parts on mine were so badly hardened that they were completely inflexible. I was shocked they've turned. What used to be rubber were more like form shaped PVC pipes.
If you were to go with all OEM, these hoses cost quite a bit and if they're new old stock they can quite possibly be degraded just as rubber bands do in storage. I just used sold by the length stuff. Cost about $25-30 including streets (45 deg) and elbows (90 deg) and hardware.
Here's the conditions of the small 2 inch long rubber section that connects the right side valve cover port to "T" fitting that goes to intake hose and PCV valve:
It's hardened so bad that its rigid and it sounds like plastic when you drop it on the ground.
Some tips:
Given the age, everything is likely brittle. Treat all intake plastic parts, especially the accordion hose assembly as if it is made of glass. Cut the hose first if its hardened and wedge the hose off with a screw driver or a spreader at the red dot while twisting the hose back and forth to protect the area in blue circle. Don't yank on it.
If the hose outer diameter isn't exactly the same as original, the snap ring don't seal well. Use good quality hose clamps and lubricate the new hose with transmission fluid. You won't believe how much easier it makes it go in.
If you can't get the tiny hoses to stay securely, fit a 3/8" hose over it and hose clamp it so you won't have a vacuum leak. There's 3 or 4 (that I'm aware of lol) vacuum jumper connectors. One or two of them is hidden under the passenger side intake manifold right by coolant temperature sensors.
