Wow..that's a lot of miles on the first tranny. Well done. Agree with pawoSD. Could be clutch pack or B1 piston (less likely), but in either case transmission will have to be removed.

Try disconnecting the kickdown button underneath the gas pedal. Could be it in these 'overnight' cases.

Bowden cable and vacuum regulator will not keep it from getting into 3rd indefinitely. If it NEVER gets there at any rpm, it's highly unlikely that it's one of these 2 components.

I assume you've redlined it in 2nd? did you check the fluid? If it's going towards a gray/black, that's grit from your dead clutches.

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1982 300TD 210K miles ("The Replacement" aka "The Anvil") - SOLD
1979 300SD 245K miles (never ending project)
2007 Pinarello F3:13
1995 Ducati 916 (SOLD, sniff)
1999 Ducati 900SSie (SOLD)