Air in Fuel and hard starting .

Hi everyone , I really hope someone can help with this . I've done a two tank veg oil conversion on my 300tdi Defender . It's run perfectly for the last five years until a couple of months ago . When I switch to veg ( it's actually a veg/diesel/petrol blend ) it begins to lose power . The instant I flick the switch to diesel it runs freely again . The veg circuit runs from a custom tank to filter to lift pump to delivery valve to injection pump . The diesel runs from the original tank to filter to injection pump . It missed it's lift pump to start with but soon got used to life without it ! Here's what I have done so far to try and solve the problem . Attached clear tubing between all the key points to see what's going on . Starting the engine the veg line is initially clear but quickly begins to build up air bubbles to the point where the engine begins to struggle . The air can be seen after the filter , after the lift pump and going into the ip . So I replaced the hose from the veg filter to the lift pump , and from the lift pump to the delivery valve and from there to the ip . I also put on a new Delphi lift pump . It made no difference . I then bypassed the delivery valve sending the veg straight to the ip . It made no difference . I then checked the hose clips at the lift pump and found I could tighten one of them by a couple of turns . Problem solved ! I then drove some 200 miles on the veg blend over the next few days until I had to brake suddenly in traffic .. and as I depressed the clutch the engine gave a good shake as if about to stall and the problem was back big time . I then looked to the return line . It looked sound with no leaks , damage or loose clips . I replaced the leak back hose . It made no difference . I tightened a glow plug and a leak back bolt which had a slight ooze of fuel coming out . It made no difference . I then disconnected the return valve ( Biotuning UK ) and attached a length of clear tubing to ip return . The exiting fuel was frothy but all hoses on the delivery side were now free of air and the engine ran perfectly . I reconnected the valve and it was back to air bubbles again . Today I lifted the return pipe from the tank and placed it in a bottle . Unlike the other day when the fuel was exiting from the ip frothy and the delivery lines were clear , the fuel coming into the bottle was clear and the bubbles were back in the delivery lines ! So I'm really out of ideas now ! The problem also exists in the diesel circuit but the only problem it causes there is hard starting after the car has been sitting for some time , it otherwise runs perfectly .

Are you using proper fuel line hose clips or jubilee type clips?
I find with jubilee types in the smaller sizes that the bulk & rigidity of the screw housing part stops it fully clamping the whole circumference of the hose.
Leads to leaks out of fuel after pumps or leaks in of air before pumps.
Also damages the hose so tightening up might not cure the problem.

Ratae , it's a mixture . I'll recheck them tomorrow .One of the things that really puzzles me is how the air and bad running disappear when I pull the hose off the ip outlet but reappears when I reattach the hose at the ip and pull it from the tank .. nothing adds up .

Don't know much (anything!) about the internal works of diesel ip's but is it possible it's something going on inside the pump or the pipes you attach the hoses too?