I HAVE A SERIOUS PROBLEM!!PLEASE HELP!

i just got a 1998 chevy tahoe z71. its a very nice vehicle with little problems here and there like most used vehicles would have. except there is one thing that needs to be fixed very quickly.

my tahoe has a tendancy to stall out when going up or down a steep grade or coming to a stop. i have to pump the brakes twice for them to come into effect and when it does stall all the electrical components inside are still working. i dont know whats wrong. id like to have this problem fixed. i dont like stalling at a red light.

please give me your feed back and suggestions. i need this to get fixed soon. thank you.