Wow thanks so much for all of this information, really helpful. I have started with the dashpot and vacuum hoses and think that there may be some initial issues there to sort out. Still trying to figure out he best way to get parts if I am still not sure on the model of lift. Would you know if Yale be able identify this with the serial number if I gave them a call? Would the dashpot be easy to replace without knowing the model of the lift? Any good places to get parts for older lifts? Not sure about the engine type, would that information be on or around the engine with some marking?
Sorry for all the questions, I'm a newbie just trying to figure it out.
Again, thanks so much for helping me out.
Ian

well this serial number covered many model ranges ie: the prefix data on the serial number
E187.
This covered models:
GC/GLC 040/050/060/065 RF/TF
GC/GLC 040/050/060/065 RG/TG/ZG
the letter after the prefix designates where it was manufactured
V designates it was manufactured in Irvine, Scotland
now of course the numbers after the V indicate the unit number assigned to the truck on the production line, being how many have been produced.
The letter on the end indicates the year of manufacture
W indicating one of these years 1999, 1973. It will not be the 1973 because they did not use this serial number configuration back then and the next W serial number will not come back up till 2022.
I think it would be safe to say your lift is a 1999 model.

Now as for your steering/engine stalling problem?
These are all the possible scenario's i can think of that may effect the steering...

first i would check and make sure it is idling at the right rpm, sometimes the idle screw will back off causing the engine to idle too low which can cause the engine to stall when loaded up.
Another thing is it may have a dashpot on the throttle body that is vacuum operated and it may not be kicking in at the right time or at all, make sure it is working and is adjusted properly. Check for leaks in the vac hoses also.

Is this an LPG engine? mazda FE,F2 or D5 engine (possibly but unlikely a UA or VA engine?)
If it is LPG you want to check the mixer diaphragm and make sure it is in good condition, check down inside the throat of the mixer and clean it out if there is any contamination buildup in it. How does the engine idle and accelerate? Is it sluggish or misfire any?
Pull the vapor supply hose off the regulator/vaporizer and see if there is any oil buildup in it, it may need to be cleaned and/or replaced.
Engine timing may be retarded too much causing the engine to stall out under a load

Brake inching pedal adjustment, make sure the transmission is completely releasing when the brake pedal is in it's natural released state. Transmission drag can cause low engine idle and can cause it to stall out if put under a load when steering.

check the steering relief pressure, the adjustment is usually on the orbital valve on early models and later on it was incorporated into the auxiliary pressure circuit on the hydraulic control valve.
And lastly, the steering priority valve, this valve keeps a balance between the hydraulic operation and steering operation so you can maintain effective steering above and during using the lift/tilt/attachment operation, most of the time when this valve malfunctions it usually prevents steering effectiveness making it weak and keeps the main fluid flow diverted towards the hydraulic system, but i suppose in a rare case it may not divert hydraulic fluid correctly and keep the steering flow engaged too much on the steering side and not shift back towards hydraulic operation correctly. In other words the valve gets stuck due to dirty fluid. Early models had this valve located in the transmission area but not part of the transmission control valve, it was a separate valve mounted beside the trans control valve, later on they incorporated it into the hydraulic control valve.

I know this is a lot of information and not specifically clear but i do not have direct access to the yale database so i cannot do a serial number lookup to see exactly what this truck came with from the factory. I'm basing my information on my experience dealing with yale trucks back when i worked for them. The information i've shown above is listed from most common to least common, in that order so i would look closer at the first possibilities and only go to the last ones if all else reveals no changes.

If this is a gas truck then I'd recommend checking the vacuum operated anti stall valve or maybe check the pressure relief valve on the valve block is set correctly