I have a 2003 f150 4.6 liter , cuts out BAD when trying to hold a constant speed, to the point it shakes the whole truck. also has a loss of power when it does it. Any ideas

 

Unless you have the equip to really check out systems you generally have to use the process of elimination.
A detailed visual inspection of electrical connections and wiring is usually a good place to start looking and it doesn't cost much.
If the feeling is like you are turning off the key , it is a good possibility it's ignition system related.
If it drags like it is just losing power it could be fuel related.
The list for ign is a pretty lengthy list and if you don't want to spend a lot of $$ have the vehicle checked for codes in the memory and there just may be something there for direction.
If one coil quits you can notice a change and maybe get a check engine light. If the engine wants to quit altogether , now you need to look for what supplies the entire system with signals. To attempt to diagnose over the net is very much different from actually being there.
What is the present mileage and what kind of maintenance has been done to the vehicle ?

 

thank you hanky, I have checked all coils and vac lines , on social security takin it to a shop is out of my budget. to many gadgets . can test most any sensor just need to know where to start.

 

It may not involve any sensors. You state it cuts out bad and loses power. Something like that could also involve the fuel system. When was the fuel filter last changed? Did this symptom start right after a fuel purchase ? What about the previous maintenance?

 

the truck has 137,000 miles. the fuel filter was changed about a month ago. these systems started about a year ago just gettin worse. I've also noticed it surges you can feel it when going down the road . as far as service I changed plugs and the coilspring that goes to the plug about a yr ago rite after it started cutting out. nothing else has been done outside oil and filters

 

You are at the point where the fuel pump pressure should be checked.

 

I checked the pressure it is 32LB acording to the book 28 to 50 is ok?

 

The next thing you might check is the EGR valve for sticking open. They do accumulate a lot of carbon and it sometimes causes them to stick or hang up and stay open longer than they should. Usually if they are open at idle it will cause a rough idle or even a stall.

 

I have no vac at the egr valve at any time. there is vac at the sol. for the egr and resistance measures good on the sol.

 

There are certain conditions that have to be met for the EGR valve to operate . And as you have found , there should be no vacuum to the EGR valve until certain engine speed ,vehicle traveling at a certain speed, engine temp etc., Because some do accumulate carbon and they can hang up causing performance problems.