yeah..I have one of those, and just now finding out how common this is.. 01 firebird 3.8 surges at cruising speeds, increasing as temp. rises, and increasing load...
have replaced fuel pump/ filter, plugs, wires, ign. switch,crank sensor, o2, sensors, cleaned and tested EGR, also throttle body, and Im sure there's more I cant think of right now...
it sometimes feels more like its jumping in and out of overdrive?!,.??
was pulling a o2 code till they were changed, any ideas/ suggestions??

Unless this is in a thirdgen, it will get locked. This is for 82-92 F body cars. A 4th gen site would yield you better results. And if it is in a thirdgen, then this post should be in general engine or the v6 sub-forms.

in before lock... i know you tested the egr but this to me sounds like an egr problem. had this happen in an 85 vette i had. car would 'surge' from about 45 to 60, ran tests and concluded it was the egr valve, replaced that and the switch in it and the problem went away. at the least, you might want to take the valve off and clean all the passages to it. good luck!