Considering any ‘real’ changes to the engine vice false indications, still start the search at the front of the aircraft and work back.
Is the engine anti-ice switching in the overhead panel (left side?), or on a sub panel next to the windshield – I assume that the 900 is similar to previous 125 variants.
I don’t know about the particular 900 engine installation, but some other aircraft types had auto engine anti-ice activation as part of the anti-surge protection, but I can’t recall any 125s being fitted. Thus, although this is unlikely to apply, this or a similar problem may result in the anti-icing being auto-activated at altitude with thrust reduction, i.e. top of climb. Check thrust lever quadrant switches.
Consider air cond valves hi/lo auto switching. However, with failing memory, this may only apply to older 125 variants, and electrical activation is still required – St Elmos fire is the key. Are the air valves switched for the left side of the overhead panel? If so back to bonding and stray electrical paths / faulty switches (subject to shorting in high static). I knew of one aircraft where the autopilot would disconnected anywhere near a Cb.
Electrical problems may also depend on who / where the completion was carried out. Older 125s were predominantly A or B models, with greater variability in the A (USA/FAA) market.