My 1998 GT, automatic, is having a random misfire. But no trouble codes are issued and my certified mechanic is stumped trying to find the cause. It has well over 200K on the odometer so we did the obvious regular maintenance items (i.e. fuel filter, belt, hoses, plugs, wires, coil packs, etc.) but the problem still persists. If it's in gear, foot on the brake, at idle you'll feel it in seat of your pants every few seconds. In motion at around 45-50 MPH it's noticeable and increases with speed. We've removed each injector and tested them and all 8 pass. Fuel pressure is good as well as tested fuel pump; all check out okay. Put an infrared thermometer to each cylinder with no noticeable difference in any of them. The car is factory stock. I've had it since new and this is a new issue. I'm stumped. Any help or insight would be greatly appreciated.