I have a strange issue and was wondering if anyone else has ever had it. When driving at 40-42 mph my Montero (95 SR auto) will go from 1750 rpm to 1250 rpm then back up within a second. Its almost like a hickup? It started doing this after i removed the butterfly shaft in the upper intake and blocked off the end. I have checked for vacuum leaks and havent found any. Could this be an idle air control valve or throttle body issue?

That's right about the lockup speed for the lockup torque converter - may be locking and unlocking...

What is the lockup torque converter and is that fixable?

I also replaced the spark plugs and wires if that helps.

It's not broken, that's just how it works. The torque converter is a fluid connection between the engine and the transmission, replacing the clutch in a manual transmission. The slower it goes, the more it slips. It's want lets the engine run while the truck is stopped in gear. A lockup converter has a small clutch in it to eliminate the slip at speed, which is roughly 300rpms at 3krpms engine speed. It's for economy, to eliminate the mpg lost to tranny slippage. The clutch locks up at about 42-45mph. In some trannies, it will unlock if you step down on the gas (a GM 700R4, for example), like a semi passing gear downshift. In the montero, I don't believe it does that, but if you drop below the lockup speed, it will unlock and the engine rpms will rise by 300-500rpms in the speed range you are quoting. It is also possible the tranny is downshifting, as the o/d gear is quite high for that speed, and the tranny really wants to go to 3rd. Manually downshift to 3rd at that speed and report rpms...

Is it possible the cable from the throttle to the transmission is out of adjustment seeing that he had the throttle plate off?