If you know of a friendly local garage, they might be willing to fault find the vehicle without charge (charging you for the repair/fix), but most want to charge for such work.
There could be many reasons for such a loss of power – with the occurrence being intermittent, fault finding could be tricky. I would be reluctant to pay a garage to investigate the problem, possibly paying them money for nothing.
Given the value of the vehicle, I’d be tempted to live with the problem until the next repair bill results in the vehicle being scrapped.
Beg/Borrow/buy one of these – although no warning light comes on, it might identify the fault.
https://www.ebay.co.uk/itm/2018-Universal-Car-Fault-Code-Reader-D900-OBD2-EOBD-CAN-Diagnostic-Scanner-Tool/122028314392?hash=item1c69744f18:g:risAAOSwKOJa3d7G