your first problem sounds like a classic intake manafold gasket problem.
http://www.geocities.com/endre_rl/isuzufaq.htm#faq5
You can do it yourself (parts cost under 40 bucks), or take it to the dealer and they will do it for about 300.00
your second problem could be a plugged fuel filter. Since you add gas additive at 70k for the first time you may of dislodged some junk in the tank. Again, the part costs about 15 bucks and you can change it out yourself in about 30 minutes.
-Ryan