low power rough idle ect when cold!

[mustangz]

Hello,I have just run across my first idm failure but have more problems and would like to know where to look next.I do quite a bit of computer diagnostics on all types of vehicles but am still learning on powerstrokes.It had dead cylinders on bank one and it was the idm.New problem is with the idm replaced it runs pretty bad,low power rough idle until it gets good and warm.It runs pretty good when it gets warm,about normal for 275,000 miles.Only thing I noticed that i did not like is while you are driving it and get on the throttle the volt gauge goes down just as if the glow plugs are activated but they are not.As soon as you let off the throttle it will come back up to normal.I am worried that their is some kind of problem and the idm is drawing this extra power or is that even possible?I know the gauge is correct as i had my laptop going down the road with me and it dropped voltage also.Any info would help.Oh also it has a brand new alt on it also.Thanks