DR350 missing on downhill

Hi fellow SSS guys,

I need some advice, my '93 DR350S tends to miss, it feels as if the bike is braking, when going downhill, it always happens when I'm off the throttle (not accelerating, just cruising), any advice?

My DR is also fitted with an aftermarket muffler, don't know what brand as it has no markings. She also tends to back-fire every now and then.

Any advice/help would be appreciated.

Thanks,
Dale