My 2003 s-type v6 is running very rough on start up and perfectly fine once it's warmed up. On cold start it shakes a lot and the rpms dip below 500 which turns on the battery light. The voltage also jumps around from 12.7-14.4. it's also in open loop was thinking it could be the MAF any thoughts?

 

Rather than guessing, perhaps checking the condition of your battery/charging system (you have omitted your location, which is no help to those who will try to help you); here in the states most auto parts outlets will check your charging system and run a load test on your battery free of charge.

After making sure the battery and charging system is in good shape, a diagnostic code reader can be employed to dig deeper. Please be aware that many "el cheapo" code readers attach incorrect definitions to the OBD-ii codes read from a Jaguar's engine management system (and other sub-systems too).