I have a gmc jimmy 4x4 and when im in first gear I don't have any horse power also my oil pressure goes from 350 to 275 just by stopping Also there isn't a problem with my horse power in any gear above 1st that I know of.
It is normal for the oil pressure to drop some when you come to an idle. However, the transmission has me confused. Do you lose most power in 1st only? Are there any sounds associated with the problem?