Your trigger adjustment screw has backed off. adjust back in to preferred length of trigger pull. then adj small center screw for your let off. you may have to put some locktight on the threds so that does not happen again. To answer your other question, yes lite pellets will damage the piston in that perticular gun. nothing under 7 grains is recommended for that gun.
I am having exactly the same trouble you have got a problem with a part called the sear my dad is the proud owner of a panther ro72 also known as a diana mod 5 made in 1972 (which is the original model your gun has been based on).I am trying to search for a replacement part and am having extreme difficulty if you find any suppliers let me know in the mean time i will continue my search I hope this is helpful to you your sincerely Rhys george.
