Bucking Problem

[clt1dpw]

Got a 91 Explorer from my father.
He just rebuilt the Trans.
Got it in Feb and ran great.
Now when the outside temp get above 75 degrees, and the engine has warmed up, at about 1500rpm the engine 'bucks'.
Tach goes from 1500-2000 rpm to 0 rpm and then back up. Feels like I riding one of those mech. bulls.
Had a fellow tell me that the spark plug Wires looked frayed.
Replaced them today and problem did not go away.
Has anyone come across this before?
If so, how did you fix it.

Thanks in advance.

 

[clt1dpw]

Replaced coil pack.
Did not help.
Any clues?

 

[MrShorty]

I'd probably start with a few basic diagnostics: pull codes, check fuel pressure, check for vacuum leaks, check for misfires.