Engine light power loss

So done some welding on the motor today new front bumper, just done a short drive 10mins and the check engine light flashed bout 5 times and power/revs droped off for couple seconds each time!
I had negative terminal disconnected for few hrs while working, should i have disconnected positive aswell?
Its got an immobiliser could it be that/ fuel soleniod?
Any ideas much appreciated as to wot need to look at tomorrow
300tdi auto

Anyone with and ideas?
Drove it today only flashed up twice so not a major issue just strange how wasnt doing it before.
Read loads about the spider issues on search but that seems to relate to engine cutting out .