Sounds like you could try filtering your LaNO3 solution right before use.

Whenever I use heavy metal solutions for EM I remove any microcrystals that may have formed by using a syringe filter with at least 0.2 micrometer pore size (the smaller the better). After an hour or two the microcrystals reform, but you can filter the solution again.

Dissolution of La(NO3)3 in water should be done at least at 313k to avoid crystal formation.

Thanks Holger and Mourad!