90' Golf TNA transmission 3 speed

Hi everyone,
my transmission slips a bit for 20 -30 sec only when outside is cold and the
car sits overnight. otherwise runs just fine ,shifts up ward and down no
problems, no grinding noises. when weather warm no problems. I replace the
fluid but didn't make any difference. I've read somewhere long time ago,
there is way to do some tighten the screw up or clean the breather I'm not
sure , to fix the problem. any ideas will be appreciated.