van runs great, but sometimes at highway speeds it starts to "hicup" and jerk and if i push down the gas pedal it will backfire. so i take it out of overdrive and that seems to correct the problem. and now its happening more and more. what's wrong??

 

welcome to DF...

check the fuel pressure when it's doing the bucking... it could be dropping off causing this. should be 49 +/- 5 psi.