Half engine light BMW 1 Hatchback (E81)

@user_1212754 sounds like classic VANOS solenoid or valvetronic motor playing up mate, especially with the intermittent nature you're describing. when the ECU detects a fault with VVT it throws the car into limp mode as a safety measure but if it clears and everything works fine that screams electrical or sensor issue rather than mechanical damage
had a similar thing on my mates 318i with the same N46 engine few years back, would randomly lose power on startup then be fine other days, turned out to be dirty VANOS solenoids, cleaned them with brake cleaner and problem sorted for about 6 months before he eventually replaced them

the fact clearing the code fixes it temporarily is a big clue, if it was actual mechanical timing issue you'd have consistent problems. check your wiring and connectors first around the VANOS solenoids and eccentric shaft sensor, corrosion or loose pins can cause exactly what you're describing

which specific VVT code are you getting though, that matters a lot for diagnosis. is it 2A82, 2A87 or something valvetronic related like 2851 or 2A59? VANOS codes point to solenoids, valvetronic codes usually mean the motor or eccentric shaft sensor

i reckon its the valvetronic motor tbh especially if power loss is on startup when engine is cold. these motors can develop internal faults where they work fine when warm but struggle when cold or vice versa, its a heat related failure
my cousins E90 320i had this exact problem last year, would start fine sometimes then other times total limp mode with no power. took it to a specialist who tested the valvetronic motor function with proper BMW diagnostics and found it was seizing up intermittently. replaced the motor and eccentric shaft sensor at same time since you have to take half the engine apart anyway and its been perfect since
the thing with valvetronic faults is they're really hard to diagnose without proper BMW software like INPA or ISTA because you need to command the motor through its full range and monitor actual vs target position. a generic code reader will just tell you there's a fault but not why. if your mechanic cant pinpoint it id suggest taking it somewhere with proper BMW diagnostic tools who can do live testing
also worth noting that cleaning VANOS solenoids can help short term but if they're already causing intermittent faults they'll likely need replacing eventually anyway, same with the eccentric shaft sensor, they dont tend to get better with age 😅

another thing to check is oil level and condition, valvetronic and VANOS systems are super sensitive to low or dirty oil because they're hydraulically operated. if oils not been changed regularly or its low that can cause all sorts of weird intermittent issues

from what you're describing with it working fine after clearing codes id say 80% chance its either dirty VANOS solenoids or dodgy wiring to the eccentric shaft sensor. try cleaning the solenoids first as its cheapest fix, if that doesnt sort it then start looking at sensors and wiring

honestly mate with intermittent faults like this you really need someone with proper diagnostic equipment to monitor live data while the fault is happening. if your current mechanic cant identify it take it to a BMW specialist or independent with ISTA, otherwise you'll just be throwing parts at it hoping something works

seen this loads of times on E87 120is, usually ends up being the valvetronic motor or eccentric shaft sensor. the motors about 200 quid and sensor maybe 50 so not too bad, labour is the killer though as access is tight. get proper diagnostics done first before spending money 👍