I just recently purchased a 98 F-150 5.4L. When I test drove it it had no power and the engine studdered violently. When they repaired the truck they told me it was bad coil packs causing missfires. 10 days ago I picked the truck up running smooth and strong. Then yesterday it just started doing it again. This happened right after I went to get my car washed I was wondering what do you think could have caused them to go wrong so quickly, and how can I tell which ones are bad so I can change them out?

A scanner should tell you which cylinders have bad coils. Look for any codes.

It might be your spark plugs and not your coils causing misfires--replace with the Motorcraft plugs that came in the truck. Don't try other brands.