I have one of these that I built from two sets.
They are at least as well built as the Motorola and make a good picture though unbelievably cramped.
http://www.vintagetvsets.com/emerson49.htm
Jitter is not a normal characteristic of the 7JP4, at least not a good one. It's possible you have some leakage in the base of the tube, the high voltage might be arcing and cause havoc with the picture, (the socket could also be faulty).
I have a NOS tube that had a bent wire in the base, it arced so badly you could see it through the glass. I was able to fix it but it was scary taking the base off without breaking any wires.