300SE not shifting into third

[danwatt]

1990 300SE with 722.351. First gear and second gear pull wonderfully, and the 1-2 shift is nice and solid. However, once I am supposed to be in third gear, it just revs around like it's hunting for it but can't find it.

So far I've replaced the k1 springs, the B2 piston (old one was fine but put the new one in for good measure), vacuum modulator, filter, tranny mount, flex disc, and put in new fluid. Any ideas?