2002 Isuzu Rodeo problem..

I have a 2002 Isuzu Rodeo that has a new battery..new alternator and a new positive wire cable. What have been happening is I will drive about 10 yards and when I give it gas it just revs. I had my transmission looked at before and the guy said it was fine but you never know like most mechanics will say. I called another mechanic out to my apt and he said its the alarm causing your computer to not relay the information to the transmission to know when to shift. I have a aftermarket Denso alarm and I never heard that a alarm couldve done that. The check engine light is on and I have to replace the egr valve. I also had a whole dianogtisc(sp) done and nothing came up that the transmission needed to be looked at. Any ideas on what I should do next?

does the shift indicator line up correctly on the dash and on the shifter? if not the range selector could be back and could be causing your problem.