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The stalling will be caused by voltage drop over the cable run from the PSU to the mount. A giveaway will be the red LED on the mount flashing whilst slewing. Under normal tracking conditions the voltage drop won't be too bad, but when doing a goto slew, both drives are working hard, and along with the other devices being powered you're probably drawing 6A or more at that time.
Now, if the cable from the PSU to the mount has 0.5 ohms of resistance, then the voltage drop will be:
E = I x R, where:
E = Voltage drop
I = Current draw
R = Resistance
So,
E = 6 x 0.5 = 3 Volts!
That means your mount will be trying to run off 9 volts, at which time it will stall.
With the EQ6 Pro that I have, if a stall occurs, the slew buttons then become inactive, and to get proper control of the mount back you have to do a power cycle.
Your solution to stalls will be to either put the PSU very close to the mount so as to have a very short DC cable run, or, if the cable run must be a reasonable distance (mine is 13 metres) then you need big DC cable from the PSU to the mount. Big speaker cable etc just won't cut it, and buying a specific DC power cable by the metre in a size that will be effective will be around $30 per metre (in my 13m case). What I did was grab an old 25m 240v exension lead, cut the ends off, cut it in half, and use all three conductors from one run to be the positive, and all three of the other run to be the negative. This gets the resistance over the run down to a very low level, and even pulling 8A over 13m I have no problems with voltage drop any more, and thus, no stalls!
Cheers,
Jason.
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