I had this problem occasionally on my 69 RS prior to reworking the system last year. When I took the door assemblies appart, I discovered that a couple of plastic bushings that guide/support the shaft that the doors articulate on were missing at the bottom location on both sides. Probably just wore out over time. So one possibility is shaft binding due to worn/missing bushings. Additionally, a little moly spray lube is helpful as well.
I've also noticed a tendencey for the doors to stick open if the vacuum hose routing isn't exactly as it is supposed to be. On the '69 there are a couple of large u shaped clips that bolt to the front of the radiator support, one on each side. As the vacuum hoses run down the radiator support to the actuators, they are supposed to be captured by these clips. On mine, if one of the hoses slips out, it can interfere slightly with the movement of the door in the open position - and actually press against the door cover when the door is open - and hold it in the open position. A slight push on the door would release the binding condition with the loose hose and the door would close. I assume that the '68 setup is similar. If it is, be sure the vacuum hoses are routed/secured properly.
And as a final thought - check valve ok in the main vacuum line from the intake? Relay valve working properly? Anything that might affect the correct vacuum routing at all times to the actuators.