96 Celica GT Convertible spindle nut

I bought a used 96 Celica Convertible with 85,000 miles on it. After 2 months I got a grinding noise at low speed from the left front wheel. Checked the brakes-all Ok. With the whell off I had play on the rotor. Surmised that when turning the rotor would shift causing the rotor to grind slightly on the pad as it was no longer true vertical. Took off the cotter pin and retaining nut and got about 1/3 of a turn on the spindle nut. Replaced the keeper nut and cotter pin and the noise stopped. 2 months later the noise is back and the nut did not come loose yet I have play again in the rotor where I had none after the original tightening of the spindle nut. I can continue to tighten the nut but I guess eventually I could have catastrophic failure of the suspension/wheel. Brakes/steering as something is wrong and I have no idea what could cause this as I have never heard of this before. Do not want to get screwed at a repair place, but certainly don't want to get screwed by a larghe oak tree or telephone pole either. Any suggestions? Thanks in advance!