Re: 2003 ford focus missing at 50mph
If you haven't had a tune up done in a while (air filter, plugs, etc) it's time to do so. I'd do this every 40K for best performance.
If the vehicle has any engine codes (light on), get them scanned to help narrow down the problem. It could even be related to a clogged injector. However, you first have to test that it's RPM, not speed related.
To do that, keep the transmission in a lower gear (2nd for example) and drive 30-35mph. RPM will be in the 2500-3500 range. If the 'miss' occurs around a certain engine speed (not road speed) then it may be a bad/faulty injector.
'11 Ram Quad Cab 4x4 SXT 5.7L HEMI (stock); 48,500m
'06 Pontiac G6 GT 3.5L 220hp; 64,000m
'12 Chrysler 200 Limited 3.6L 283hp; 28,000m
'99 Taurus 3.0L 2V Vulcan 145hp; 154,300m - Traded
Amsoil in all vehicles!