Re: 2000 Focus slow idle, Stalling shifting to gear
Was a tune up done (new plugs, etc) when the engine was put back together? Tune up should have been done around 100K if it hasn't been done yet, you're WAY overdue. If that's been done, then you can try a couple things. There are sensors (tps throttle position sensor) and IAC idle air controller (you said you changed this) that can gum up and cause some driveability problems, such as poor idle, gear hunting when driving steady speed, etc. I'd look into replacing those, both can be done by the do it yourselfer in about 15min with common hand tools. A repair manual also helps (Haynes or Chilton). Another one is the MAF sensor, located in the air intake tube. It can also be changed relatively easily.
Another test if the sensors don't cure the problem, is to run the tank near empty, and try a few gallons of 91 octane fuel to see if that helps.
'00 Durango R/T 360ci 290hp (modded); 138,500m
'06 Pontiac G6 GT 3.5L 220hp; 44,000m
'12 Chrysler 200 Limited 3.6L 283hp; 13,000m
'99 Taurus 3.0L 2V Vulcan 145hp; 154,300m - Traded
Amsoil in all vehicles!