i bought a 97 f-150 with the 4.2 the guy i bought it of thought he droped a valve as the truck backfires and runs real bad at idle ...untill i drove it home and once it is at high rpms it runs good. had it went fron 60- 90 in no time...now i dont know alot about engines but if you droped a valve there would be no compression in that cylinder and it would skip through the whole range of rpms right?so maybe a stuck valve or sumthin? any ideas ? thank you

 

Check to make sure you have no vacuum or intake air leak, especially around the MAF.
Air entering the intake stream will throw things all out of whack because the air the engine is seeing is not in keeping with what the MAF is reporting and things just don't work right in that situation.

Also, if it has never had new plugs and wires, I'd invest the time and money.
Motorcraft are, IMHO, the safest bet.

Good luck and keep us posted.