A500 vibrations

I've got a strange thing going on with my A500 trans; if I let go a little on the gas when it shiftes from 2:nd to 3:rd I get vibrations & they become a tiny bit worse when I click on the overdrive & then it stays even when I'm let the car go down slow. But(!) when I keep on axelirating throu the gears it runs smooth & is happy on highway use too.
I've been under & felt the outgoing shaft as much as I can while driveshaft still attached & it's tight as conctete.
Just did oil & filter change & driveshaft is well balanced + good u-joints.

Any ideas..?