Motorola ts4j

[Electronic M]

Try a different modulator, and try reducing signal to the set as well as increasing it (IIRC contrast is IF gain+video gain so try changing contrast first*).
* best to do this with a DVD or some repeatable content that does it bad so you can see how the signal level affects it....Also don't worry about watchable brightness or contrast during this test, as long as you have enough to tell if it is rolling that is all that matters.
If signal is too strong or too weak then it will fall outside the designed range of the sync clipper resulting in weak sync or sync with noise (which can disrupt lock).

If it looses it under the same conditions regardless of signal then the sync sep and osc stages should be suspected.