89 500SE engine surge

[mercman123]

This is an intermittent problem which has been noticed over the last week or so. Prior to that the car was running great and I’d covered over 600 miles the previous weekend.
Basically the car starts first time every time from cold or hot and idles smoothly.
When driving, the car will suddenly, without notice take off. This happens with or without pressing the accelerator pedal. The engine surges to 5,000rpm, I brake, then the engine dies.
The car is a UK model and I’ve noticed the ASR light comes on when the problem occurs.
I’m quite handy mechanically and don’t mind working on the beast as long as I can find the things to check, so may need some guidance there.
Any ideas what to check?
Thanks in advance