OK... First of all I may be wrong.. if so please correct me..
Normally a car when it's converted to Gas there are 3-4 important things to check.
1. Plugs
2. Air Filter
3. Hi tention Cables
4. the Air & GAs Mixer
Most of the time we don't feel the need to check 1-3 when driven on petrol... the only difference we feel would be a increase of fuel consumtion... but when converted to gas... the system becomes sensitive... old plugs, the hi tention cable ( i was told that on normal conditions this should be changed after 100000Km even on petrol)...all these needs to be attended.
Air filter needs to be cleaned very often...
Misses could be due to any of the above... too much of Air Flow in the mixer, Plugs,Cable,
Did the guys who did the convertion tune the car???