What is wrong
 

1998 rodeo. 80,500 miles on it.. Few weeks ago I change the upper and lower intake manifold gasket. I used the fel-pro kit.. Still leaking.. And it was the second kit that I bought for the thing.. With the first one didn't work. So I returned it .. This second one was the same way.. Should I only use Isuzu parts only on this...

I'm new to the whole Isuzu thing but not to working on vehicles. Both surfaces have to be perfectly clean then you need to check with a straight edge to make sure there is no warping. You are working with all aluminum parts so a little heat can warp those parts. Also make sure you are tightening the bolts down in the correct sequence and correct torque specs. I hope this helps. :)

+ 1, tightening sequence is key to avoid warping. I would have to check manual, but these thing are low torque, like inch pound not foot pounds. Did you follow the tightening sequence and torque specs?