Have a 1990 Bronco that at low speeds the engine surges and rpms goes to zero and dies. I have replace the EGR and IAC and tuned up the engine but the problem persists. What else could be causing this problem?

 

Possibly carbon buildup at throttle plate; take off throttle body and clean. Check all vacuum hoses first, one might be off or cracked.

 

Try having your catalytic convertors checked. One of them might be plugged solid, causing the problems you described. I had a 300 I-6 in my F-150 that did the same thing. After $1,00 in new parts (sensors, and other parts I was told to replace) it still had the same problem. I had my convertors checked and sure enough, the rear convertor was plugged solid.
The place to call would be a muffler shop. They would probably do it for next to nothing, I know the one I took mine to only charged me like $5.00

Hope you find out what's causing it, I know mine almost drove me nuts.

 

Check your Idle Air Controller if you have one as well. It's bolted on to the side of the throttle body. It controls in it's own little sick way how much air enters the trottle body at idle. Ford IAC's are very cheap. I replaced mine with an aftermarket brand(can't remember what name) to get rid of the extremely high idle my truck had for a long time after I let off the gas. Alternatively, you could just disconnect it but I think the IAC sends some sort of signal to the torque converter.