1991 Chevy Silverado chev truck idle problem

Engine Performance problem
1991 Chevy Silverado V8 Four Wheel Drive Automatic

I have a 91 chev 4x4 truck that had a 305 engine in it. A rebuilt 350 was installed by a certified mechanic. Before the install it idled fine. After the install of the 350 it would idle eratic and usually stall at stop signs. Also in the winter cold weather I would need to press the accelerator pedal to get it to start. The mechanic couldn't find a problem. I took it to a different mechanic and he replaced idle air control valve, temperature sensor, egr valve solenoid and cleaned the throttle body. The truck idles ok now, it doesn't stall anymore but I think it could be better. The truck when at normal running temperature and at idle will idle fine for about 5 to 10 seconds then all of a sudden the idle speed drops to a point that I think it may stall then it revs up and surges then settles out and idles fine for about 10 seconds then this drop and surge repeats about every 10 seconds. Cold weather starting is better as I don't need to use the accelerator but it cycles between high idle speed and then drops almost to stalling for about a minute, then it stays in high idle until it warms up. Any ideas on whats going on here. Also what is your opinion about changing the fuel injectors with the 305 to 350 engine swap? Is it required? Thanks for you help