Very sluggish, low power on start up.

I have a 1991 fuel injected GM 366 on propane. It was running excellent until a few days ago. It has very little power on start up.
After a few seconds on flat ground, it picks up speed quickly and seems to run fine as long as I am in a certain rpm off idle. It starts and idles good once it is warm. When it is cold, it idles a little rough, but not too bad. I am getting a code 43 which means ESC curcuit problem. I had installed a new knock and O2 sensor. I had measured the voltage signal from the ESC module to the ECM and found it to be a steady 9.2 volts. The book says between 8-10 volts is good. I was thinking the ignition timing is in full or partial retard mode thus making it sluggish until there is less load on the engine? I was wondering if the ignition module which receives the signal from the ECM could be causeing this. Any ideas?
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