1997 chevrolet tahoe not starting?

ok i usually answer the questions not ask them but i'm at the end of my rope here. i have a good battery good starter good alternator all wire running to them are also good. truck will crank as it should but will intermittently stop cranking for a second about every 3 to 4 seconds of crank time. i have spark at all cylinders and it is a strong spark. can't tell if i'm getting fuel as the motor uses a sequential fuel injection system (sfi) however i have removed the intake tube after the mass air flow sensor and shot starting fluid into the throttle valve. fuel pump is humming fine and about a week ago i check the in-line pressure and it was 61 psi which is in the range of operation. anybody else have this problem before? any suggestions are appreciated. on another note when the truck will start it runs fine smooth idle and no idiot lights. wtf!!!