2001 Volkswagen Bora Sudden "braking" whilst driving

This is a weird one which has confused both VW dealers and the local "expert" shop here in Berlin, so you may be my last hope!
For the past three years or so, at very irregular intervals (sometimes every day, sometimes weekly, sometimes not for several months), when I'm driving along, I get a sudden "braking" sensation even if I'm accelerating normally or have my foot off the accelerator or brake. The effect is almost as though I'm skidding on wet leaves - even though the conditions are dry - and it seems that the ABS kicks in. It happens at all speeds, meaning on motorways I'm having to drive very cautiously - and seemingly totally at random. The noise which is produced in these situations appears to come from the passenger side (left-hand drive!) Under the bonnet or further down, closer to the wheel.
When this happens, most of the time the yellow/orange warning light comes on. Either intermittently or remaining on for the duration of the journey. This is the light which is a triangle with an exclamation mark inside it and a near-full circle with an arrow at the end - also ESP/ASR.
What is happening here? When it happens, it's a real shock and obviously could provide some dange, although the vahicle remains pretty stable for the short duration of the "block".