Hello guys wondering if anyone got some input on this

Vectra c 3.2 v6 with LPG conversion 105000 miles on clock

Bit of history
First seen car march last year for full service and thermostat change
All ok until few weeks later he reports intermittent misfire

After looking again suspected air leak he kept returning for me to look at it

In the end I told him need someone to investigate further with smoke machine I can't find fault so he went to a vx specialist they fitted new plugs again new genuine coil packs and new inlet manifold complete.

So didn't hear from him 4 months or so so I assume it was all ok after that

Then 23rd December I fitted rocker gaskets for him
He comes back few weeks later saying he's getting spanner light and fault for O2 sensor

It was bank 1 sensor 1 range/ performance
After looking it was found sensor being a bit lazy

Fitted a second hand genuine sensor for him retested all ok

Fast forward few weeks he's saying its misfiring
Basically misfired on first cold start codes for cyl 1&6 misfire

Turn engine off an re start and all ok

He's saying its since fitted sensor tried to explain sensor isn't used when cold

I haven't got a scope

Anybody any possible ideas for what it might be?
Weird that key off and back on again and fault is ok