Engine revs pulsing and higher than normal after throttle assembly change

#1
I have recently had a mechanic replace the throttle assembly as the revs kept dropping low or the car would stall intermittently and this was his solution. Since then the opposite now happens. The revs are sitting at around 1300rpm and I am going through fuel like there's no tomorrow. The car is also quite difficult to drive slowly as it jumps as each gear is introduced. Could this be just another fault with the new (used) part or is it more likely to be a sensor issue. Any advice would be gladly received. Ty in advance.
 
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