I recently noticed some issues with my 1990 F150 4.9 I6 2WD MT. When I am idling, it will shake at around 500-800 RPMs. If I rev it past 1500, the shake stops but will resume at and through 1100 and lower. Just did new plugs today. Oil changes on schedule every 4k-5k miles along with air filter. Also, when I am cruising between 50-60, I get shakes there as well unless I am constantly on the gas and manually kind of easing on and off. Not sure what is causing these issue. Another issue: the starter is only 8 mos old but is wanting to just free spin when I try to start. Not every time, but sometimes. Last issue: when I am in 4th or 5th gear and want to accelerate with the pedal more than 45-50% of the way in, I get revving and have to back off. New to the forums, so I figured i'd drop it all at once and see what happens.

 

Might have a plug wire breaking down for the miss and buck,any check engine light?Is the starter still under warranty? either starter drive hanging or bad spot on flywheel. The reving in 4 and 5 sound like clutch slippage.If its catching high,make sure you don't rest your foot on the clutch pedal,doesn't take much.

 

I had a plug boot arcing under side to the heat shield - burned a hole in the heat shield. Check your spark plug wiring configuration too for cross fire.

 

Are you saying the clutch is slipping in 4th/5th?

I'd pull the PCM and open it up to look for bad capacitors… these things are failing at rapid rate due to age.