Recently my 2002 4.2 Coupe has taken to revving up and down slightly on light throttle openings, for instance at 30mph. It also appears to change up and down a gear. Could this be engine or gearbox problems? Also having drawn a blank trying to find a security module either new or secondhand,does anyone repair these?

 

Could as easily be a split or loose (vacuum) hose. Go hunting or find an expert.