Intermittent shift problems

My 904 transmission sometimes does'nt want to shift from first to second gear when in drive.

When it sticks going from first to second, I can shift from drive to neutral and then back to drive and it will shift fine until the next time it's parked. After that it may or may not shift correctly the next time I drive it.

Is this a linkage issue, or do I have a valve or something sticking?