99 f150 4.2l 4x4 reg cab long box. truck starts up fine runs perfect in park . go down to reverse ( with foot on the break not going anywhere) the engine labors but still runs. neutral runs fine . drop right to 1st it labors more . 2nd the same . when you put it in drive it stalls immediatly . just cleaned tps , throttle body . checked all vac lines for leaks ... none . any sugestions?

 

Clean or replace the IAC and MAF sensor.