SONY CDP-H3700 not(always)playing discs correctly

[OXAUDIO]

This early '90s machine sometimes 'judders' and mistracks about 1.5 to 2 minutes into a disc, although it plays correctly through to the end after that.
I have replaced the KSS240 laser, the loading belt, and the spindle motor(this last item had an intermittent fault which caused it to need a 'helping hand' to start, and a DC resistance of ONE OHM rather than the usual 11, though it reverted to about 10 ohms after I removed it!). The laser assembly is free to slide along it's shaft, the gearing, cabling, sled motor, etc., are all OK, and the discs in question all play OK in other players. When fitting the new spindle motor-one of the usual 'mabucci' type-I ensured that the turntable was fitted as accurately as possible. I'm reluctant to adjust the focus or other presets, since I have never before had to do this when replacing lasers. Being currently without a working Oscilloscope, I am unable to check waveforms anyway.
Anyone have any ideas as to the cause of this problem??

[gortonandy]

You probably need to tweak the tracking gain very slightly clockwise. Move the laser assemble by hand to the end and you will see two yellow pots underneath. RV101 is the tracking gain