Does this problem hapen all the time or intermittently?
Do the locks keep clicking toward lock as if you are commanding power locks or do they cycle from unlock to lock and bac and forth?
ok, you said it is intermittent. Does this seem to be related to temperature?
Does this happen after you lock or unlock the van?
Do you use your key or the remote to lock the van?
Any history of accident damage on the van?
The first time this happened it was last fall, cool and moist. It didn't happen again until about 3-4 weeks ago (spring, cool-moist), but it is now happening even when hot and dry. It will happend when I unlock using either the remote, key, or auto-lock/unlock button on the armrest. Lately I've only used the key because it re-locks immediately after trying the remote so only the key is effective when immediately openning the door.
There have been no accidedents with the van.
Ok a few thoughts come to mind here.
First and most comon would be the integration relay under the dash that controls this sytem, it's toyota's earliest form of body control systems, I have seen these internally fail or become victim of moisture and corrosion and cause issues such as this.
Second items to check here is to be sure that the key unlock warning switches are connected and properly operating.
I do not suspect a door lock switch at this point based on the conditions you have stated however be sure to check that the switches don't seem to stick and that they operate normally.