I recently had a similar problem on my 8.1 gas engine, took about 5 minutes to get up to 50 mph on level ground.
No check engine light appeared. Very intermittent...
My Workhorse dealer replaced both oxygen sensors and the fuel filter. Cured the problem.
2004 Monaco La Palma 36DBD, W22, 8.1, 7.1 MPG
2000 LEXUS RX300 FWD 22MPG 4020 LBS
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