[old_tv_nut]

The detector diode is not the problem here. It either works or doesn't. Since you have a picture, it's working. If the problem exists with a good video feed, then you need to look at AGC and sync separation.

1) Did you try the fine tuning? Tuning too high in frequency can make video detail overemphasized and affect sync. Also make sure it's not so far off towards lower frequency that you could be receiving the lower sideband of your modulator by mistake.

2) Try adjusting the AGC down and up from present setting to reduce/increase the signal level/contrast. If the detector output is too large or too small it could cause sync problems.
3) If the AGC adjustment doesn't improve things, wait until you know you have good tubes and then proceed with trouble shooting.