It really sounds like it's time, as stated by Paul R. (and Welcome Paul-nice to see another pro out here
) Did it all on my '88 'TE last year (same motor as your car).
Since your car left the factory, the valve guides have been improved 3-4 times, increasing longevity/durability. And by virtue of an M103's valvetrain design, they do work the valve guides hard. The valve seals are better, too, and yours are 12 year old rubber at this point.
The only other thing I'd recommend is changing your coolant hoses-the thing's apart and your cooling system is open, so bite the bullet and they are one less thing to worry about for the naxt 150k or so
Best of luck, and enjoy the renewed car (watch-it'll even shift better)