Not revving past 3000rpm, not changing gear

[Gareth_X5]

Hi everyone, I'm hoping someone is able to help me. For the past couple of weeks my car has been intermittently not letting me go past 3000rpm and subsequently not changing gear. I've researched this for a while now and have come up with a couple of things that fit, they are; the nylon gear in the transfer case actuator servo is worn (but there are no warning lights on dash) or the fuel filter is overdue a change.
The thing is though I have this problem for maybe 3-4 days and then for a couple of days it's absolutely fine. And then back to the 3-4 days of crap again. I am able to get up to normal speeds when the problem is there but only when being extra gentle with the accelerator. Slightly too much and the car bogs down, won't Rev any further and will not change gear until you back off again. When it is stuck in this situation the car shakes back and fourth as though hitting the redline. I haven't changed the parts I have already mentioned yet as I was hoping to get people's opinions on here first before diving in. Sorry it's quite a long post but any help would be greatly appreciated.

Gareth

[kkx5]

" the nylon gear in the transfer case actuator servo is worn (but there are no warning lights on dash.."

[X5Sport]

Check all your vacuum hoses for leaks.  If the engine isn't getting enough turbo boost then that could limit you.

Could be a bad MAF, bad injector too.  The transfer box is behind the gearbox so why would that limit the rpm and gearshift?  kkx5 may know with his expert knowledge of this area, but it puzzled me.

[Gareth_X5]

Thanks for replying. I shall check all pipes but if I had a leak somewhere I don't see how the problem would come and go??

[Steamyrotter]

Does it change any different in Sport mode?

And are all gears there in Manual mode?

[Gareth_X5]

When it sport it makes no difference at all and when in manual the problem is still there but I'm able to change through all gears

[Intravenous de milo]

I had something similar but it was a coolant leak that was the cause.

[Gareth_X5]

Cheers for the input! I couldn't see any leaks at a glance but I will take a closer look tomorrow when I have more time. It's so frustrating. Currently for the last 3 days I've had no issues whatsoever. If I don't find something soon I think a trip to the garage will be needed.