I know there are lots of posts about this but there very messy. This post is to post solutions and work carried out that did not fix the problem ONLY. Please dont spam with speculation.
I have this issue and changed hoses and the MAF but the problem is still there although I only lose power around 85 MPH now which is better than 60 MPH which is where it started. There are NO error code faults found so im working blind on this but will keep you posted.
Andy
P.S when updating your post please EDIT your old one to keep this clean.
Not sure if you've sorted your problem but I wanted to express my extreme thanks to this forum for it's invaluable help in solving the same issue I had and share some of the steps that got me there. Bit of background: I bought what looked like a nice spec 2001 Silver 1.4TDI (75BHP) from a guy that turned out to have pulled the wool over my eyes about a couple of issues the cropped up as soon as 30 mins after I picked up the car. Of course he pleads innocence and will hear nothing of a return for a refund or contribution to repairs (so far). Long story, I was saddled with (amongst many, many others) the limp mode problem when putting my foot down in 4th or 5th on the motorway, usually around 3000 - 3500 rpm when accelerating to join the traffic flow (just the point where you least need a drop in power, eh?). I spent hours (days?) and money on solving this, so I'll share the areas that this forum has kindly pointed out to check and then indicate what my specific problem was:
1. Get a code reading (this is important - don't waste your time and money until you at least have an idea of where the fault might lie. I spend more money than I should at my garage so they were obliging to read the error codes on my car for free. I had one related to the cam shaft and boost pressure. Replaced cam shaft sensor (easy job on the 1.4TDI) but problem persisted)
2. Vacuum hoses from N75 relay at front right top of engine bay - checked for splits/leaks. Replaced some of the obviously worn pipes, but found no specific splits. Problem persisted)
3. MAF sensor - Didn't check this, didn't see it called out as a common related issue. I think it will be accompanied by an engine warning light on the dash, which I didn't have)
4. MAP sensor - Replaced this with a Bosch original that I managed to find for £30 at a friendly local parts store that ordered it in same day for me. Beat eBay, local VW and local parts store change both in time and at 50% of the price or less! But didn't solve the problem
5. Intercooler - this is at front left of radiator. Checked this for splits or oil leaks, but looked 100%
6. Checked large turbo air pipes to head and from intercooler and to and from exhaust manifold for leaks, found nothing obvious
7. Fuel filter - was recently replaced
8. Took out the heat shield above the CV joint to get a better view of the turbo. Saw more oil on and around turbo than I expected and pipe leading from intercooler into turbo looked a little loose. Checked where the vacuum pipes run into (out?) of the turbo and found (more by feeling than by sight) that one of them had completely popped out (there are two) and the rubber hose at the attachment point had stretch so it was no longer a tight fit. Also removed the air intake pipe mentioned above and found the o-ring had perished (cross section was square not round any more and rubber was hard). Replaced this and re-fitted pipe. Test drive - SUCCESS!
9. If none of the above are the problem, it might be the Turbo. If you have a variable vane turbo (not on the 75BHP 1.4 AFAIK), check that mechanism for movement.
I think that is about it, hope it helps someone!