If it has sat for several years I would start with fuel system myself.
Maybe the filter got gummed up?  Injectors need a cleaning? Have you used
any seafoam since starring it?

Maybe go through all  the small vacuum hoses and check the air intake boot
for cracks? I don't know maybe it's running so lean it only equals at
higher rpm? These guys are pretty knowledgeable so will steer right.

Good luck!  And good on ya for resurrecting the family van!

Ryan
Asheville,  NC