Turbo problem...dealer replaced valve...still

If you have a problem that isn't caused by something obvious, you need a Ross tech VCDS cable. This is a laptop computer diagnostic cable to talk to the car's computer. Without it you cannot do the more advanced tests.
Note about generations - some generations have similar engines: Mk3= 1996-1997 3rd generation Passat TDI or 1996-1999 3rd gen Jetta TDI Mk4= 1998-2006 New Beetle, 1999-2005 Jetta, 1999-2006 Golf, 2004-2005 Passat TDI Mk5= 2005.5-2010 Jetta TDI Mk6= 2010+ Golf TDI
Remember, an engine needs fuel, air, and compression to run. Low power is related to a lack of one of these or a sensor problem making the computer thinking there's a lack of these. Any sensor problem could also be caused by a bad ground or broken/chaffed wire so also check every section of the wiring of the suspect sensor for breaks.
Bad MAF sensor - very likely cause on the mk4 TDI. Not common on the mk3 TDI (1996-1999 Jetta/Passat). Early mk4 MAFs failed often.Error codes normally do not show up with a faulty MAF since the signal degrades instead of going out completely. Through VCDS, checking MAF actual vs. specified at idle, high rpm, and high load will quickly show a bad MAF or other problem causing a low MAF reading.
Clogged intake manifold - carbon buildup chokes the intake manifold, starving the engine of air. Only ultra low sulfur diesel is sold in North America now so there should be much less buildup in the future. Always use good quality synthetic engine oil on your TDI..
Anti shudder valve shut or almost shut (does not apply to mk3 TDI, more for mk4 TDI) - there is a spring loaded valve right before the intake manifold. Newer TDI use an electronic valve and are not as susceptible to sticking. If there is excess carbon buildup, it could shut in a partially closed position.
Clogged snowscreen/air filter - a clogged air filter will starve the engine of air. A clogged snowscreen (large debris air pre-filter) shouldn't block off all air unless the aux-intake flap is also clogged.
Clogged fuel filter - change interval is 20,000 miles but biodiesel use (cleans out old buildup) or bad fuel could clog it early, resulting in fuel starvation. Algae or bacterial growth in the fuel tank could also clog the lines.
Boost leak - a cracked hose or loose connector lets measured air out. No air or major leaks = poor engine running or stuttering. A visual inspection may not reveal all the possible or hard to see spots where leaks can form.
Hose inside ECU (mk3 TDI only, does not apply to mk4 or newer TDI) - this hose leaks and normally sets a check engine light,
Vacuum lines to/from turbo and n75 solenoid - these dry out over time and crack or can rub through. It's possible they are clogged. The n75 solenoid controls the turbo wastegate or VNT vanes with either vacuum or pressure. b4 Passat - on firewall above coolant reservoir, a3 Jetta - on pass side near air box, a4 Jetta/Golf - on firewall above brake fluid reservoir.
Problem with the n75 solenoid, VNT actuator, VNT vanes, or vacuum lines. You should have already checked the vacuum lines, the below test will inspect the entire system. Start the engine and through VCDS, click on "engine"-->"measuring blocks"-->hit "up" until you reach "group 11". Compare Specified vs. Actual MAP. This compares what's actually happening and being observed from the boost sensor (barring a faulty sensor/plug/wire) to boost the computer is requesting (what should be happening). They should be relatively close. If they are far off this normally results in limp mode but it could also be contributing to the problem. If you have a mk3 you have a conventional turbo but you can still use this test to check the n75 solenoid, the wastegate, and vac lines. However, wastegates are much less susceptible to sticking vs. VNT vanes. The videos below show how it works. The lever on the outside is welded to a lever inside the turbo housing. This is how it moves the VNT vanes. See the below videos to see how smoothly and free the lever should move. It should not stick or bind at all. Vacuum is being applied to the can, not pressure.
If the test shows poor response or no response at all, it could be sticky VNT vanes/actuator (mk4 and newer TDI only), The vanes or actuator can stick or fail to function, the lever should move freely.
If the actuator is fine, also check the n75 solenoid and vac lines. The n75 solenoid controls vacuum or boost to the vacuum line going to the turbo wastegate/VNT actuator. To test, apply voltage to the solenoid or swap with a known good unit. If you have a mk4 TDI, you can swap it with the EGR solenoid to test. Also check the plug for corrosion and the wiring harness for chaffing. If those are good, disconnect the VNT actuator rod and move the vanes by hand. If the vanes are stuck then remove the turbo and clean the inside of the exhaust housing to free the stuck vanes.
Faulty injection pump's fuel injection quantity adjuster - these are occasionally set wrong from the factory or after seal replacement. It's also possible the fuel pump's internal quantity adjuster is faulty. Applies to 1996-2003 TDI only or TDI that use a Bosch VE injection pump (not pumpe duse or common rail). Injection quantity should be 3-5 at idle and up to 36-38 at full throttle.