I don't have the car/manual right now, and memory is hazy, but there is a process by which the trunk unlock can be overridden, by inserting the key and turning it the opposite direction. it might be possible your trunk switch is tweaked in that direction, causing an intermittant malfunction - but probably not.

The whole system operates on a central vacuum source, so if the trunk or one of the doors doesn't lock/unlock, it's very likely due to a faulty vacuum actuator, or mechanical resistance/friction in that actuator mechanism.
It works when you lock/unlock it rapidly because it temporarily loosens it up by going forward/backward. You should be able to track it down by:
- Hit the unlock and see which switches unlock first and which unlock last. The slowest lock wiill be the weakest or stickiest mechanism. Check the vacuum line to the trunk lock if it's slow.
- insert the key in the trunk and work the mechanism manually to see how sticky it is. Zap it with WD40 and see if it helps.

DG