the car was hardstarting so i replaced the camshaft positioning sensor... on 1990 Ford Thunderbird SC

now the car bucks and surges but it starts fine now

Your check engine light must be on. What are the trouble codes?
there is no trouble codes check engine light is not ran great before i changed the cps but it was hardstarting...i changed all the plugs,wires and vacuum leaks were detected...all the old plugs were evenly worn no oil on them..compression was good on all 6 cylinders...i unplugged the cps and now it runs good again but is hardstarting again...and i do run premium gas in it possible i bought a defunct cps
1 more answer
Start with the basics, spark plugs good, no vacuum leaks,perhaps check compression is even in all cylinders, look at the old spark plugs see that they are all the same color, none are mechanically damaged or oil fouled.