limp mode imo would be a speed limiting feature like nothing above 25. what youre describing sounds like a vary large vacuum leak and the engine just wants to die. egr is only active under certain conditions like cruising and throttle usage less than 30% (cant remember the exact numbers but its close to that) something that can happen is carbon can get stuck in the egr causing it to be stuck open and that will cause what your describing. sometimes you can remove and clean the egr, also check for other obvious vacuum leaks
also test the map sensor
http://forum.73-87chevytrucks.com/smforum/index.php?topic=29996.0M.A.P.
(Manifold Absolute Pressure)
Symptoms of a Defective MAP
The 'Check Engine' Light is On
Surging
Rough idle
A rich fuel condition, which may cause spark plug fouling
Detonation due to too much spark advance and a lean fuel ratio
Loss of power and/or fuel economy due to retarded timing and an excessively rich fuel ratio
A vacuum leak will reduce intake vacuum and cause the MAP sensor to indicate a higher than normal load on the engine. The computer will try to compensate by richening the fuel mixture and retarding timing -- which hurts fuel economy, performance and emissions.
Testing
Check the reference voltage supply at pin C of the MAP sensor (gray wire typically).
Measure voltage between pin A (ground) and Pin C (+5 V ref) with a DVM. It should be right around 5 Vdc.
With Key on engine off the MAP voltage (Pin B, light green wire typically) should read around 4.8 to 4.9 volts. If it reads the same as pin C the sensor is probably is history.
Also check ground reference pin A. With everything off, measure resistance between Pin A and electrical ground (stud next thermostat). It should be fractional ohms
Manifold Vacuum
kPA 0 10 20 30 40 50 60 70 80 90 100
Hg 0 2.9 5.9 8.9 11.8 14.8 17.7 20.7 23.7 26.7 29.6
Volts 4.9 4.4 3.8 3.3 2.7 2.2 1.7 1.1 0.6 0.3 0.3
W.O.T. Idle
Manifold Pressure
kPa 100 90 80 70 60 50 40 30 20 10 0
Hg 29.6 26.6 23.7 20.7 17.7 14.8 11.8 8.9 5.9 2.9 0
Volts 4.9 4.4 3.8 3.3 2.7 2.2 1.7 1.1 0.6 0.3 0.3
W.O.T. Idle