Ok, so paid a guy to swap intakes. From plastic dormant. 4.6 to oem aluminum. Just didn't have time to do myself. We'll after he cranked it had dead cyl. 2 washers lodged in # 4.. all that's fixed now.. runs like sewing machine now. Only issue is idle. Little high. Bout 1200 rpm. But come to stop light. After 8-10 seconds it'll drop a bit.. 8-10 sec later it'll drop again . But not enough for ecm. Getting P1507 code. Have new IAC. and new TPS and pigtail.. sprayed all around looking for vacuumed leak. None found. I'll be happy camper after I get this worked out. Any ideas??

 

That's a specific code and lots of info out on the web to resolve it.

Look through here and see if something helps

https://rerev.com/obd2-codes/p1507/

 

Seeing sort of the same on my 98 with the 4.2. and 100K miles......once in a while idling at a light the RPM drops significantly momentarily, then comes back to normal.

Recently, let the truck sit a few days without starting it and it started up then this issue happened almost instantly, but it seems to recover.

Place I bought the truck from says the coil pack module may need replacing?

I dont know when the last tuneup was, so I may try adding some STP fuel injector cleaner.