1999 Mercury Sable w/V-6 (Duratec 24 valve engine

Trans issue or engine misfire?? While starting off, occasionally has a hic-cup where engine misses or trans slip (5-10 MPH. On a steep incline, inching forward feels like a nisfire. Engine light on but issue was months before light came on. 2 mechanics tried to find problem. Light on was a trans light code. ANY IDEAS???