Loss of power when towing

Hi Just purchased a 1987 Defender 90, took it to a music festival last weekend near Inverness and towed my caravan. The thing is as soon as I reached any incline (crawler lanes were out of the question) The revs died on me and I lost power. I had to stop anbout 20 times and it took me over 4 hours what should have been less than 2. When in 4th gear revs got high and lost power so I would drop to 3rd and then it would cut stall. Every time I pulled over (and anoyed half of Morayshire) the Defender would start up again an pull of. Here is the catch. I put half a bottle of redex into the tank and I think this has losend up lots of crap into the fuel system - so I took it to a local garage (while still at the festival) and they replaced the fuel filter and bled the fuel pipes. On the way hom I would say there was about 70% improvement it only cut out 4 times on very steep hills. Has any one got any other suggestions, ideas? Should I change the fuel filter again and again until it stops? will this help? Seems to run fine when not towing. Thanks for any advice you can offer.

I would say fuel starvation, in this case I would start at the tank...

I would drain the fuel tank - there will be drain plug on it, if its the side mounted tank its on the back edge.

Get all the fuel out, and check it out, if its gunked up I would filter it out and wash the tank out (with fuel, kero etc...) before putting it back in (if its a little bit of fuel just use it for a bonfire and put in fresh stuff), then clean out/change the filter again and bleed the system.

Is the pump clogged up?

Or the injectors blocked?

Keep changing the filter if it's getting blocked up?

Just booked landy in to garage to have tank drained and cleaned. I will let you all iknow the result. Cheers for the help