I have a 1990 series 2 rs turbo....recently been tuned, and setup, by local rolling road specialist, also had new injectors.

Basically took car out today..and when driving, and dropping down gears, when revs are relatively still high..ie 3rd gear to 2nd....there is popping/back fire from exhauset..this can be multiple back fires/pops..dependant on revs..

any ideas what is causing this?