The basic problem is that you have been p***ing about with a perfectly good
car.  At a guess the changes to the ESU program don't agree with the all the
other mods and the mixture is too rich.

If you want a go faster BMW then get rid of the 325i and buy a 330i or a
540i or even an M5 and DON'T mess with it.

Technically, 4:10 is too low, not too high. Higher numbers are a lower gear
ratio. I can't imagine why you would put 4:10 on a car that can't fit tires
that would need to be offset by gear ratios. But, to each his own.

You've replaced the cams, messed with the throttle body, changed the chip,
monkeyed with the exhaust, and you are asking us what went wrong? I can't
begin to guess what your trouble is.

Does the Cruise Control hold the speed properly?

Assuming you can't keep a constant speed with your foot, have you considered
that you have moved the power band to a place where speed control will be
very difficult? Let's say you still had the car is stock form, but was
driving at 60 in 3rd gear instead of 5th. The motor would be churning out
power at about 4000-ish RPM. At this speed, the throttle would be very
ticklish, more gas will speed the car easily, less gas will slow it. Now,
you have swapped the gears (increased engine speed much the same as using a
lower selection in the transmission would do), altered the fuel delivery
system so that it is more sensitive, and altered the exhaust so it can allow
the engine to respond faster.

I can't see any other result than the one you got. Put the car where the
engine is running along at 3000-ish RPM in 5th gear, and see if the speed is
easier to maintain. In my car, that would be about 70 MPH, in your car it
will be more like 55-ish. Since the power curve has been moved, the gas
pedal will be far more responsive than in stock form.

Another unintended consequence of your modifications should be that the car
is now slower than it was before. The engine will reach Red Line long before
it gets to the top ground speed that it should be able to attain. The car is
electronically capped at 130-ish MPH. As it turns out, that is also very
close to the mathematical top speed of the gear ratios. When the car is
running at 130, the gears dictate that the engine will be running very close
to redline. Now that you have altered the gears, Red Line will arrive before
130 is reached, resulting in a slower car. The engine will have better
response because the power curve is moved, but will reach it's top speed
earlier in the car's progression down the highway. Your changes would be
good for the track, but I'm not sure they will be what you really want for
the street.

If your foot keeps speed okay, but the Cruise Control varries too much, then
I'd suspect the cruise control's linkage being loose. The cruise pulls the
linkage to speed up, then holds steady, except that the linkage is mal
adjusted so the car slows, then speeds again, then slows, and repeats.