I don't see a Stern "Tristar" machine in the database here or IPDB
I'm guessing a "classic Stern" so ...
for the high score thing probably means the battery is dead on the MPU ... remove at once before any corrosion starts and ruins your MPU
sticky flipper , is it hitting the playfeild or do you have any ideas what may be sticking in the coil sleeve perhaps ?
a thread "bump" either way