my W.A.G.; his battery is crapping out, and will not provide enough current to run.
Load test the battery (measure battery voltage 'at rest', then under hydraulic stall [lift to maximum , then read while holding the lift for 5 seconds]) if your battery looses more than 10% of it's at rest voltage while in hydraulic stall, you have found your problem.
Now rather than blame it on the service tech, we have to blame it on the operator, for not correctly maintaining the battery. (and that was about a 6 thousand dollar bit of failing to bother to properly maintain your battery)
This MAY be an early e50z, and it may have only 1 DC hydraulic motor and a load sensor and priority valve to provide power to both steering and lift functions, and it may have worn out the brushes in that 1 single motor. (is it getting much worse noticeably?)