Picture goes in and out
This will require you opening the back cover of the television to test some component with the use of an ohmmeter.
This may be a slight drift - like someone is messing with the fine tuning
or such a substantial change in tuning frequency that the channels go by
as though you are surfing.
Possible causes depend on tuner type:
1. Quartz tuner (10 button direct access digital synthesizer) - For a
slight drift, a component is probably changing value, possibly the
crystal in the reference oscillator. For gross changes - flipping
through channels - it is more likely to be a digital control problem -
the microcontroller is misdirecting the synthesizer to change frequency.
2. Varactor tuner (buttons but not direct channel access) - If only a
single pushbutton selection is the problem, the the varactor tuning diode
for that button is probably changing capacitance. If all channels in
a band (Vl, Vh, U) are having a problem, it is more likely to be a
drifting D/A or faulty AFT (Automatic Fine Tuning) circuit or power supply.
3. Turret or switch tuner (Knobs) - A component like a capacitor is changing
You will have to get in there with a heat gun or cold spray and track it down
the old fashioned way. At least, the problem is almost certainly localized
to the tuner box (and possibly the controller if applicable).
As noted, gradual slight changes in tuning are likely due to frequency
determining components drifting.
Uncontrolled channel surfing is probably a logic problem. For the
quartz tuner, this could still be marginal connections causing the
microprocessor to misdirect the synthesizer to change channels.
For the latter case, particularly, the cause may still be bad connections
resulting in loss of channel memory and/or erratic behavior.