what could it be , its not the gas cap. was told code P1456 -

Question

On hwy, van will hesitate /shudder / jerk; letting up on pedal and gently easing back onto it will
correct the problem for a bit. Also stepping on pedal and shifting to lower gear seems to minimize at
least feeling the shuddering. Especially noticeable cruise is on. Cruise and A/C on seem to make it
worse.

Answer 1

P1456 Evaporative emission control system leak detected

When the vehicle hesistate and shuttter and letting up on the gas pedal to correct, this is a ignition misfire with sparkplug wires.When you use the a/c you add a load to vehicle so this make it a definite misfire, sparkplug wire and sparkplugs should be replaced..As for P1456 check the purge hose which is right near the EGR this hose tends to rupture and set P1456