I have one of these (the 150b) and it seems to work for a short while before the display changes to 5E5Exxxx (xxxx seems random). Even after powering down and back up I get 5E5E0000.
I have a massive 208 page 150C manual, but I don't see a mention of this.
edit: Oh, if I press all three switches down before the unit fails it shows CCCCCCC6, which I think indicates mode 6. I also can't seem find out what mode 6 is...
edit 2: Three filter caps on the power supply (100uF 50V) were dead. Measured 72uF 4R3 ESR, 42uF 69R ESR, and completely open circuit. After replacing these with 220uF 63V low ESR caps, the display is less flickery, measurements seem more stable, but it still stops after a while with 5E5Exxxx. However, the three terminal regulators seem VERY warm. Are they shutting down? There's more caps in the power supply, I think I'll replace them all. I'm thinking that the device is either drawing too much current or the switching regulator providing an input voltage to the regulators is running at too high a voltage...