misfire on 4 and 5

[tripleD]

ok guys ive had the best of the best gm mechanics work on my wifes 99 4x4 tahoe!! need some answers PLEASE!! runs perfect when drivin normal but get down on it it will jump up and go then goes to hell shakin like crazy!if u hold it the check engine light comes on but at the same time it clears up! my scanner and everyone elses scanner says misfire on 4 and 5!sometimes u can get on it alittle it gets up to about 50 to 55mph the shakin begins but hold it steady and it will clear up! it has completely been tuned up all censors changed u name it ive changed it! guys im at a dead end i need help bad cause u no if mama aint happy well she makes me unhappy so help me please!!