TRVs on flow or return?

Classic symptom of one being used in the 'wrong' direction.

That is a symptom of a TRV on the return. Most bi-directional TRVs simply have small holes restricting the flow so the 'shuddering' you describe is less likely to occur, although one (the Randall(?) RAS-C Revolver) lets you reverse the flow through the TRV body if this occurs.

In your case your options are (a) move the TRV to the flow end of the rad (b) turn the TRV body through 90 degrees so it's aligned with the flow (which means the head will protrude horizontally rather than standing up vertically) (c) replace with a RAS-C revolver, adjusted appropriately (d) replace with any other TRV, which may be less likely to judder.

The message from John Stumbles contains these words:

The closer TRV heads are to the heat the less notice they take of room temperature in general so a horizontal head on the return gives the best chance of restricting changes in room temperature when outside conditions vary.

Many modern ones claim to work equally well on either end. However I found that with a TRV on a return (new install, no easy way to know which was flow and return in advance - 50/50 chance ;-) it whistled as it closed down, so had to swap it anyway.

But this can be a short-lived advantage if there are small children around who stand on the handily-placed TRV head as though it were a step :-)