Jimmy is cutting out around 20 mph and around 60/70 mph??

If that doesnt work i would check the mass airflow sensor

a common problem with these vehicles is the egr valve any mechanic with a brain should have told you or checked it.If you can get it out yourself you might just need to clean the carbon deposits out.This valve lets exhaust back into the intake and if it gets stuck open it will let to much in causing it to bog down or soun lik it is cutting out.