F 150 ignition problems

hey all, new user here-i'm having a prob w/my '98 f-150 that my mechanic (who i trust completely) is stuck on. it's a 4.2 liter V6, 5 speed manual tranny. 130 K miles and living in hot and humid Florida. The problem is the number 5 cylinder misses when the engine gets hot. the check engine lite starts blinking and engine runs very rough. Engine runs fine after cooling off- every time. My mechanic has replaced the plugs, wires and ignition coil with no effect. it's not losing any coolant and there is no sign of water in the oil (head gasket ?) HELP!

has he checked the compression? could also be the fuel injector not working once it gets hot. also once it starts to run rough use a old plug to verify it has spark to that cyl

hey thanx--my next thought was to check the compression, but it seems awfully strange to have compression loss when the temp goes up ?? i asked my mechanic about the fuel injector but he doesn't think that could be the problem.

Crappy design in the exhaust gas recirculation EGR on this engine. It has it going through small individual tubes that are part of the manifold that easily clog up. Then bumps, vibration, whatever will dislodge the buildup and it goes into the combustion chamber. Usually either cylinder 6 or 5 as they are closest to EGR. Many times it will either start coating the plug to short it out or a chunk will lodge in the plug to short it out. The chunks are problematic as they may dislodge on their own after setting the code or removing the plug dislodges the chunk leaving it in the cylinder and the plug looks fine when it comes out. The manifold can be taken off and then those tubes need to be picked and scraped at to clean out the build up. First take off the EGR and see if there is buildup indication inside the intake. No coating and this might not be your issue. But right around those miles is when it usually happens.