miss fire

I recently started having misfire problems with my 99 Tahoe. I bought it for really cheap because it had what seemed to be a blown head gasket. I replaced the head gasket and intake filters and the misfire went away and for 6 months the check engine light never came back on except for an o2...

bad miss fire HELP PLEASE ok, when I put into reverse or drive I can here a slight backfire in the tail pipe and it seems to struggle when I give it any gas. I have changed spark plugs, wires and coil pack but, it still does this. it also has really bad gas mileage now, can anyone help.