my truck has 5.7 litre engine and has developed a slight vibration which can be felt in steering wheel and floor. not always , but sometimes if i drive until its up to full operating temp, turn ignition off for a couple of minutes and then restart it will run smoothly for a while but slowly the vibration returns. any ideas. tia.

 

Have you checked it with the decoder yet?
Or checked by a engine analyzer at a repair shop to see if one the spark plugs is not firing properly?
When was the last time you did a tune up?
Perhaps start pulling out the plugs to see if any are wet or fouled or eroded.

 

i did put new plugs in but it made no difference, the old ones were clean and dry. no codes are are showing any problems. the fact that it does run good sometimes after a hot restart is what baffles me. mileage is between 14 & 15. i haven't taken it to a shop as yet but it may end up being necessary to isolate the problem. thought i might try a new fuel filter though i doubt this is the problem, i think it must be electrical related.

 

Try replacing the EGR valve. Sometimes you just have to rule out the simpler things. Also, how clean is the air filter? I've seen some so dirty that they effected the air flow to the engine and thus its performance. There is an outside chance the Oxy sensor could not be reading correctly and thus effecting the engine too.
There comes a point when taking it to a Prof mechanic is a the logical thing to do.