Anyone gotta clue? TMP36 Pi PICO.

I have a PI PICO W which I am developing code for amongst which is a TMP36 temperature sensor
The code I have reused form other thermostat projects but its suddenly giving the odd false reading.
#define CHIP_TEMP_IO 28
#define CHIP_ADC 2
#define RELAY_IO 21
#define IN 0
#define OUT 1
#define ON 1
#define OFF 0
void initialise_hardware()
{
adc_init();
adc_gpio_init(CHIP_TEMP_IO);
gpio_init(RELAY_IO);
gpio_set_dir(RELAY_IO,OUT);
}
void set_relay(int flag)
{
cyw43_arch_gpio_put(CYW43_WL_GPIO_LED_PIN, flag); // set the lamp
gpio_put(RELAY_IO,flag);    //set the relay
}
float read_temperature()
{
static int index;
static int flag;  //initialised to zero at startup
static uint32_t rawbuf[32];
const float conversion_factor = 3.3f / (1<<12);
float result;
float last;
int i;
uint32_t raw;
adc_select_input(CHIP_ADC);
raw = adc_read();
//wtf?
printf ("Raw=%0X\n",raw);
result=((float)raw)*conversion_factor;

return (100*result-54);   // the pcb warms them up by about 4 degrees
}
I am reading the temperature every two seconds as in
while (1)
{
sleep_ms(2000);
flag=(flag? OFF:ON);
set_relay(flag);
temperature = read_temperature();
printf("Temperature is %.1f C\n",temperature);
}
about one time in ten the output jumps to about 20°C less than expected e.g.
Raw=3BE
Temperature is 23.2 C
Raw=3BE
Temperature is 23.2 C
Raw=3BE
Temperature is 23.2 C
Raw=29B
Temperature is -0.3 C /************************
Raw=3C0
Temperature is 23.3 C
Raw=3C0
Temperature is 23.3 C
Raw=3BF
Temperature is 23.3 C
Raw=3BF
Temperature is 23.3 C
Raw=3BF
Temperature is 23.3 C
Raw=3C1
I have added decoupling to the TMP36 supply which is fed from the ADC 3.3v supply which seems to be a steady 3.29V according to my DVM.
Do I have a bad PICO W? TMP 36 or what?
Other PICOs with the same chip don't do this...
The only difference is that this PICO is running different and a lot more wifi/server code
Is it possible that some interrupt is - er - interrupting the ADC read with dire results?
I cannot but think this is a hardware issue.
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Anon.