Clio Engine Problems?

1998 Clio 1.2i

Engine seems to be misfiring, due to custom exhaust can hear popping much more frequent than ususal, always on decelleration, sometimes while holding speed.

Also on opening bonnet a kind of coughing sound can be heard on revving at beginng and quieter at end (only revving mildly)

Have recently had starting issues at randon, immobiliser appears to be functioning fine.

No warning lights at any time.

Any help greatly appreciated!