diagnosis help needed- transmission, fuel pump??

Hi, I have a 2005 4.1 sport x5 with 35,000 miles on it. It is doing two odd things when driving.
1. When i am driving at a speed right below 40 miles per hour or when it is in 5th gear it drives as if it is chugging ( almost as if I am consistently partially pressing and releasing the breaks). It doesn't always do this but is pretty consistent.
2. Occasionally when I start to accelerate from being stopped it slams as if someone has rear-ended me. This has randomly happened 3 times within the last month.

I would like to think that it isn't a transmission issue considering how low of mileage it has..

Any thoughts as to what this could be? I haven't taken it into a mechanic yet. This is my second BMW the first was a 2003 325i with 120,000 miles.. had the same issues.

Also, there isn't a check engine light on

hows it drive in Manual mode?

It does the chugging while running in 5th but shifts and runs fine in all the other gears

still has top end power?
spark plugs been changed?

i have a 2004 BMW x5 and have recently had a new fuel pump put in since then I have noticed a sluggish start to my ride. This is noticed when Im taking off from a stop..like a stop light..it takes awhile to get going and build up speed. This is very obvious going up hills as well. You had mentioned spark plugs, could this be a cause of my issue as well. What should i be looking at as far as price if so?..Thanks