I second the issue as being fuel related, possibly a fuel blockage/restriction of some sort. A fuel pressure test (if it's common rail....I'm not familiar with this engine, though the mention of a computer makes me think common rail) will verify if this is the case.
However, it seems more than slightly coincidental that the problems started after the alarm install. They may have removed the alarm, but may have caused a problem during the install, shorting a wire or whatever.
I know this is a rather vague answer, but as a mechanic, this is where I would look, based on the info you gave us.

Good luck, let us know if you find out more, or find the cause.

Also, welcome to the forum!

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