Problems with CMYK/LAB colors

Question

I'm independent and I send many print jobs to different print shops. I recently discovered to my horror that some color critical jobs have gone to press (to the same shop) with mismatched PMS colors. At first I blamed the print shop but they complained the CMYK values did not match. When I opened the two brochures in question, one had (for example) Pantone 288 C, color type spot, Color mode CMYK, 100/67/0/23 (dimmed because the colors came from an imported illustrator eps logo). The second (bad) brochure had the same color and color type, but in the mix it had Color Mode: Lab, L 8.039, a 8, b -50 - greyed out because again, the color came from the SAME eps file.
My next step was to remove the logo file from the bad Indesign doc. Then I doublechecked the color - success, the color now says color mode: Pantone solid coated, 288 C (which is from the built-in indesign swatch book). Then I brought BACK IN the logo file from the brochure that printed correctly. The color reverts back to LAB again!
When I import this same logo into a blank document, the colors are fine (CMYK with proper values).
I'd seen in other forums suggestions to check or uncheck "Use Standard Lab Values for Spots" in the Ink Manager, however both documents have this unchecked. (Adobe seems to encourage checking LAB but I've never done this, I'm afraid to start now as it will probably give my print shops fits.) And both have "convert spots to process" checked, which I always thought was the cure-all: shouldn't PMS 288 (CMYK) always be PMS 288 (CMYK), as long as both are converted to process? Why would one being a LAB and the other NOT being a LAB make them different? 288 should be 288....? (but I guess not....)
The good news is, the Zevrix extension "InPreFlight" will warn me if run on each document, so I don't have to drive myself crazy opening old files and doubleclicking every color. But I'm wondering what the heck is happening.
CLUE: I believe this all stemmed from a "brilliant" idea I had to shorten the list of color types to choose from by going into Indesign>Presets>Swatch Libraries and removing libraries I'd thought I wasn't using - I left all my Pantones, but I must have deleted an important one, because it was after I'd done this that I had noticed the Lab colors rearing their ugly heads. However it didn't occur to me that LAB (CMYK) would be different from regular CMYK since they have the same name. When I replaced this folder with one from another mac, viola, the problems stopped. However it doesn't stop the old, previously created files from continuing to open wrong and be printed wrong, if I don't catch them.
Any insight anyone could provide would prove most helpful. It would be nice to get a night's sleep for a change.....thanks!
FYI I'm using Indesign CS3 5.0.4 (recently updated) with Color Settings CMYK: U.S. Sheetfed Coated v2 and color management policies-preserve numbers (ignore linked profiles).

Answer 1

>one had (for example) Pantone 288 C, color type spot,
Mike, you are implying in your post that your jobs are being output as CMYK, but your colors are set as spots. If that's the case your printer is converting the spot colors to process CMYK, which would cause the problem you are describingPantone Solid colors are converted to CMYK via color management when Use Lab is checked. The resulting output values would depend on the document's destination CMYK profile.
If your job is indeed CMYK you should stop using the Pantone Solid library and use the Solid to Process library instead, which always output as Pantones CMYK builds.
Use Lab is useful when a job is to be output as Spot color because you will likely get a more accurate preview of the color and a better proof to a composite printer. Many Pantone colors are out of the CMYK gamut. Lab can also be useful if you are sending a job to CMYK and you want to color manage the conversion of a solid color to CMYK, rather than using Pantones built formulas which are device dependent.