shifting problem
 

Every time I make a hard left or take on an incline at all my truck feels like it slips into neutral the gear shifter says its still in drive but it's like it just loses power and I'm beginning to think its more of a common problem than I thought because my girlfriends 01 is doing the same thing and the guy I bought my truck from I talked to him and his 03 and 01 does the same thing I've changed the transmission filter pit on a new line because one was leaking and had new syllinoids put in the transmission but still doing the same thing and BTW if I sit at a red light it will slip in to neutral also if you guys have any ideas please help me out I'm clueless at this point unless maybe its the clutch packs???