Ouch - those ports are a nasty! They will open/close when the edges of the valve slider reaches the widest part of the opening, which is probably way off, explaining why it locks up since its likely sending air to both ends of the cylinder at once.
One thing you could do would be to make a thin valve plate with proper openings, but it requires sealing the plate to the port face thats there without leaking between the ports. Could be sealed with some gasket goop, if the plate was as large as the entire face so the studs on the steam chest help squeeze it down.
Other way would be to see if you can remake the slider to take into account the wide openings where the holes are in the slots. All depends what the diameter of the holes are, spacings, and what the eccentric travel is.