It sounds like just increasing throttle position doesn't work. This leads me to believe you are running to lean. Does the thing pop back and die if you try and rev the motor?
If so, you probably have a bad control pressure regulator (warm-up regulator). This device regulates the control pressure for the system. It's job is to enrichen the mixture during cold running. It also gives enrichment during sudden acceleration by vacuum control.
Bosch Master, ASE Master, L1
33 years MB technician