87 f150, 4.9. truck misses at hwy speeds. It feels like your running into a headwind but it is actually missing. At idle it runs pretty smoothly for a few minutes then it starts to increase rpms up and down, then it stalls. The only way to not stall is put in neutral and then back in drive.

 

Have you pulled/checked for codes?

Perhaps the IAC valve is gunked with carbon/sludge...post back with codes if you can.

Edit...Sounds like TPS could be faulting too...
All assumptions as of right now.