Normally my brother's 300SD has boost but yesterday after driving around for over an hour, it lost boost and was a bear to move around. We let it sit for over an hour while we went and looked at a 190E for a friend, which he ended up not getting because of very poor repair from crash damage. When we got back, and drove the 300SD home from my friend's house, it had boost again. What would cause this?
I am guessing heat dissipating over a long period of time heated up the spring and caused it to move and unseal the wastegate, and it needs to just be adjusted, but could it be anything else?

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Current: 1975 450SEL, 83 300D, 88 Yugo GVX, 90 300D OM603 swap, 91 F150 4.6 4v swap, 93 190E Sportline LE 3.0L M104 swap, 93 190E Sportline LE Megasquirt, 03 Sprinter, 06 E500 4Matic wagon.

It is more likely that there is charge air pressure and that the wastegate and turbo are working - unless there are other symptoms you have yet to notice.

However, boost means very little without additional fuel. The signal to the alda may be faulty. Check restrictions such as the manifold banjo and bolt and the entire section of boost signal line - remove any switches from that signal and create a direct line to the alda.

put a gauge on the boost line when you drive next.

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+1 on the alda sense line. This is the most common problem. There's usually a sense port on the intake manifold, which connects to the Alda via a switchover valve. The lines can become clogged with manifold goop, and the switchovers can fail.

If it is the Waste Gate opening too soon and adjustment does not work some have installed an Adjustable Manual Boost Controller in the Waste Gate Line.

Essentially it is a one way valve that will not open till the adjusted pressure is reached.

My Year and Model has an EGR and there is a Thermal Temp Switch. If the EGR was opening when it was not supposed I think that could reduce the Boost Pressure.

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84 300D, 82 Volvo 244Gl Diesel

Don't assume that the waste gate is open....weak fueling due to the Alda not sensing boost is the more common problem.

You could very well be correct.
The OP brought up the Waste Gate Idea. I just mentioned what others had done in that area.

The ideal thing would be to install a Gauge and verify what is going on with the Boost. If He has one or Autozone has a Free Rental of one of those Vacuum Fuel Pump Gauges He could T that into the ALDA Line and see what type of Boost He is getting in that line.

I think the main issue was that it was intermittent and the OP believed it might be temp related.
If temp related that makes it sould less like it would be a Fuel Issue.

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84 300D, 82 Volvo 244Gl Diesel

w123fanman,
Were you able to figure out the boost issue on your brother's car?