I had this at 102k miles, replaced the intake manifold gasket. It has
been good up till now. At 160k, I got the code again, same symptoms
and it appears to be the gasket again.

This time I noticed that the idle was not only affected when I sprayed
some carb cleaner around the #1 cylinder, but to a lesser extent, it was
also affected when I sprayed around the base of each fuel injector.

Do I need new o-rings? if so, what size are they? The dealer doesn't
have them in stock.

Would there be any problems with using a little Ultra Grey RTV on the
gasket? If not Ultra grey, any other recommendations? Maybe Copper
Kote?

I put some silicone dielectric grease around the base of each injector. I
started it up and it idled smoothly at around 800 rpm. I only ran it for a
minute, I'll see how it does tomorrow. If it runs good tomorrow, I'll just do
the fuel injector seals.

Sounds like its the IAC (Idle Air Control) valve.

From the factory service manual:

DTC P0507 will set if the PCM has commanded the IAC valve closed and the actual RPM is more than 100-300 RPM over the desired RPM. The amount of RPM error allowed depends on ECT.

At normal operating temperature above 80°C (176°F), the idle must be within 100 RPM of desired or DTC P0506 or DTC P0507 will set. The malfunction history will store the desired RPM as well as the actual RPM at the time the DTC is set.

If either DTC P0506 or DTC P0507 become active during a drive cycle, the PCM will run an intrusive test if all the proper conditions are met. The conditions to test are to be at a steady throttle cruise 32-64 km/h (24-40 mph) with airflow between 4-10 grams/second. When the conditions are met, the PCM will issue a motor move command while monitoring MAP sensor value.

If no MAP sensor change is detected during the intrusive test, the HIGH or LOW idle conditions detected is probably due to an IAC valve circuit problem. The idle control table of the scan tool will indicate if an intrusive test has ever been performed since the codes were cleared. If an idle problem is detected, and the PCM never ran an intrusive test, the IAC circuit must be checked for proper operation.

The idle is normal with the AC on but very high, 1800 when first started and
around 1000 to 1300 at temp. The carb cleaner test does indicate a vacuum
leak at the manifold gasket as well as at the base of the fuel injectors. I just
got a gasket and a universal fuel injector o ring kit. Hope the right ones are
in there, with my luck, there will only be three the right size.

It was the gasket, again. I used copper RTV on the gasket this time, I' not
going to be able to start it until tomorrow afternoon. The RTV needs 24
hours to cure.

The gasket had pulled through the loop around the #1 cylinder and got
sucked in. I'm using a Felpro gasket with a steel core this time. The OEM
fiber gasket just doesn't seem to be up to par. The dealer's parts guy told
me that the replacement gaskets are now steel cored.

That wasa problem with some late model S-Series. I think there's even a TSB about it.

It idled nice and smooth this morning, around 750. On the way to work, the
check engine light went out. I do hope this gasket lasts, they are really
difficult to change.

If you have to change it agian, try removing the upper motor mount and upper transmission mount. You can rotate the engine forward several inches to gain extra working space.