Watch the rotor while the engine is turning over to make sure that the distributor in all the way down to engage the gear on the camshaft.
If it is not down far enough to full engage, the cam gear will push the distributor gear up until the gears don't mesh and it will jump time. Sometimes the gears will settle back down and engage again, but it will be out of time.
Happened to me once. The design of these distributors are a little wonky and its easy for this to happen.
