2 factors to consider:
1. Idle Valve Control that may need replacing or dirt (if intermitten); or
2. Accelerator cable link is jammed and needs to be lubed or perhaps looked into. The 126, unlike the 124, does not run on cables and has links: K-jet which can be lubed.
I don't suspect an OVP problem here.
126 tailed by a 203, 129 leading the pack.