I have a 2003 Ford Fiesta Lx. 1024cc. recently I have started to get a misfire. I have replaced the plugs and the air filter. The plugs showed that the engine is in good condition.It only appears in slower speeds.Fuel consumption is good and overall its the same as it has been in the three years Ive owned it. It accelerates as it always has. Lovely car except for this misfire. It has been suggested by a ford technician that it is the Lambda sensor. My problem is that the manifold cowling bolts are rusted solid and I don't want to risk brute force in removing it. Unless I have to.This would lead to drilling out. Re threading etc etc. Is there anything else that might cause it that I could check and solve it before I go down that road. I have a comprehensive repair manual but it isn't really helping. Plug leads and coil etc also seem fine. Help would be greatly appreciated.