Bongo Fury

After some advice.
I have a Bongo with a 2.5TD which is doing my head in now.
Hooked up my car trailer the other day and drove a few miles with a car on the back and it became sluggish between 1000-2500 until the turbo kits in then its fine, even to the point where on a hill it would not rev over 1500 an got stuck would not move.
No Smoke from rear, do not think turbo as it kicks in hard at 2500.
Also add that once warm she will not fire up right away and can take some cranking to get going.
Without a trailer she seams fine BUT you can still tell that there is something at the low rev's.
The bongo has pulled this weight loads of times without any problems.

Can anyone point me in the right direction?
Already done -
New Glow Plugs
New Fuel Filter
New Vacuums all over
New Auto Gearbox Fluid (right amount where engins running etc)
Cleaned out small filter in pump
Cleaned out EGR Valve
Run Injector Cleaner threw at least every 6 months
Engine Serviced ever 6 months

My next step is I have a spare engine so going to remove the injectors and send them in to be overalled.

It would be worthwhile checking the filter in the fuel tank and the (long shot) fuel filler cap valve, and the wastegate operation.
Good Luck...

Also worth checking the brakes on the trailer aren't binding. They would tend to get worse as the heat builds up which would explain the drop in performance. What is the combined weight of the the car and trailer?

check the little pipe from the air inlet manifold to the vac advance on the top of the fuel pump, this pulls up the diaphragm in the pump top.
ive seen these full of oil so the diaphragm does move too well.

or the nose seal or side seals on the pump. put your hand under the fuel pump------is it wet.