Does anybody know why my vauxhall omega 3.0 elite 1996 is backfiring threw the inlet manifold and wont rev?

THIS ONLY HAPPENS WHEN ITS WARMED UP.IT WONT REV ABOVE 2500RPM AND BACKFIRES THROUGH THE INLET MANIFOLD.IVE CHECKED THE EXHAUST FOR BLOCKAGE ,IVE ALSO CHECKED VARIOUS SENSORS E.G.(CTS /THROTTLE BODY/ AIR TEMP)BY PUTTING A RESISTER IN PLACE OF THE SENSOR TO GIVE THE IT A (COLD ENGINE READING BUT TO NO AVAIL IS THERE SOMETHING THAT I AM NOT CHECKING OR DOING THAT COULD FIX OR AT LEAST PINPOINT THE FAULT/S.?? I HAVE PUT NEW PLUGS AND LEADS ON THE CAR ABOUT 6 MONTHS AGO AND REPLACED THE GEARBOX OIL AT THE SAME TIME/BECAUSE THERE WAS SOMETHING WRONG WITH THE BOX IT KEPT COMING UP ON THE DISPLAY "AUTOMATIC GEARBOX" AND GOING INTO LIMP HOME MODE. I DONT KNOW IF THIS HAS ANY BEARING BUT I THOUGHT IT WAS WORTH MENTIONING.THANKS IN ADVANCE