B3 Idle help

[MarkCamVW]

I wonder if someone could help me with a problem with my 1996 1.8L petrol estate with Bosch Montronic fuel injection. After 175000 miles of trouble-free running, the engine suddenly stared revving itself at idle. I replaced the Idle Control Valve (VW 048 133 031) which fixed the problem immediately. However, 2 days later the revs went high while I was driving, and are now stuck at around 30000rpm. Any suggestions as to what this might be and how to fix it would be gratefully received. Thanks. Mark