it is important to keep the correct size vacum hose on that MAP sensor line, the MAP pressure has to be within a certain range for it to work correctly. Also make sure it is conneted to the correct port on the throttle body, MAP Sensor line should be on the bottom port and the top port is for the Vacum Advance on the Distributor.
In the service grams on this system they mention many times about making sure the vacum lines are not damaged, cracked or connected incorrectly.
Another issue they mention about this same symptom is an incorrect dwell setting, incorrect dwell setting will force the system into open loop and not accelerate, also the dash MIL light should be on in this condition.

And to properly setup this S15G LPG system you should use the laptop program, it would be very difficult to get all the settings within specs without it.

As for trying to replace that ECU?
good luck with that, they did away with that vacum operated MAP sensor and went to a total electronically controlled ECU MAP function.