94 g20 electrical

well first off let me start by sayn hello. i have a problem with a 94 g 20 van it shows on the ohmmeter that it's charging but when u drive it with the headlights on it dont want to shift if u turn them off it shifts fine. the amp gauge in the van shows 9 and below when u turn the heat and lights on but ohmmeter reads 13.9. thank for any help.

A good voltage check at battery and fuse box would be first step in repair.
If it is affecting shifting see if the PCM is losing voltage .
Is the speed dropping out?