I have an '89 Fender Strat Ultra. It came with Sperzel locking tuners. They've worked beautifully for 20+ years. Recently, occasionally, the high E string will slip and drop half a step to a whole step and once it's done this, it will not tune back up to pitch. If I replace the string it will functional correctly for a while, days, weeks, then slip again. Once it slips there is no returning that string to pitch. Is it time for a replacement tuner? Got a line on where i might get one tuner?Is there any other fix? Thanks!