Your problem is that the tx is not crystal controlled (not the stereo
crystal), or that it is crystal controlled and its PLL is being jammed
heavily by something you supply.

Similar problems occur with non-crystal-controlled FM transmitters where
the receiver's PLL goes crazy trying to decide which channel you're on.
Since stereo means larger bandwidth (on higher end receivers).

You can try in this order:

1) Receive in an inexpensive stereo FM receiver at some distance (10
meters or more). If it's ok you need to cut the Tx power, you are
saturating the radio.

2) Find and disconnect the Tx PLL and see if the problem goes away. The
station will drift but a good receiver should be able to lock onto it for
a couple of seconds. If this helps, you may need a trimmer on the Tx
crystal to pull it to an exact frequency that the receiver PLL likes.

3) Force mono operation on the receiver, and confirm that the problem goes
away. Then go to stereo, and on the Tx try to change the level of the
19kHz carrier. It is likely too high or too low (more likely).

hope this helps,

Peter

PS: For such a low distance you should have 1mW or so of power. The Tx
probably has a hundred times more. This is WAY too much.

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