I had the same problem on a 601 engine. It drove me crazy for a year.

When you have the problem, pour water on the IP to cool it. Carry a couple of gallon jugs.
If that solves the problem, you need to rebuild or replace the IP.

The person who told me about this is a heavy machinery diesel mechanic. He said that they started seeing this after the introduction of ULSD.

I bought and installed a used IP and the problem was solved.

This may not be your problem. Might be a high probability that it isn't. But, pouring a couple gallons of water on your IP is cheap and easy.

__________________
Greg
2012 S350 BlueTEC 4Matic
2007 ML 320 CDI
2007 Leisure Travel Serenity
2006 Sprinter 432k
2005 E320 CDI
1998 SLK230 (teal)
1998 SLK230 (silver)
1996 E300D 99k, 30k on WVO
Previous:
1983 240D, on WVO
1982 300D, on WVO
1983 300CD, on WVO
1986 300SDL 237k, 25k on WVO (Deerslayer)
1991 350SDL 249k, 56k on WVO - Retired to a car spa in Phoenix
1983 380 SEC w/603 diesel, 8k on WVO
1996 E300D 351k, 177k on WVO