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Your main problem is almost certainly that the hydraulic system is stuck in high-pressure mode, which means the engine is continuously dumping a lot of power (which gets converted to heat) into the hydraulic system irregardless of whether the hydraulics can use it or not at any point in time. The major malfunction is most probably a clogged orifice in the regulator/safety valve block (rear, inboard valve block where the supply pipe for the power steering comes from). When this orifice gets clogged, the flow of hydraulic fluid which unloads the pump gets blocked and the system is continuously producing high pressure whether it needs it or not. In your particular system, it's also possible that the priority valve for the power steering (located in the same valve block ) is somehow malfunctioning and continuously loading the pump, but this is much less likely. The other question is where the crud which plugged the orifice came from. Probably the screen in the hydraulic reservoir has a hole in it (and may be completely collapsed which is not uncommon in this model). Unfortunately the 300/350Us require draining the entire hydraulic system and disassembly of the reservoir to check/clean/replace this filter (i.e., it's not as simple as the 340U apparently is). If you do this, you have to lift the top of the reservoir STRAIGHT up or you'll destroy the filter and end up having to replace it whether it really needed it to be replaced or not. If you go the cooler route, you "might" fix the symptom, but almost certainly won't cure the disease. If your pump is continuously operating at high pressure, and especially if it's doing so at high temperature, it's not going to last very long; it's will be quite expensive to replace. Good luck whichever route you decide to pursue.
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