I'm no expert on FR trems - only got one guitar with a locking trem and that an Ibanez version of one. But any instability is normally to do with the pivot blades not being sharp enough or notches occurring in the pivot screw edges. You said that you bought a cheap FR style trem, and these are the areas where the cheap ones suffer as they often use non-hardened steel for these parts. If you want a good FR trem, unfortunately you have to spend the money either for an original or else a very good licensed copy.