Vintage Air A/C problem

[baddaw]

hey guys , i have an 1980 ls1 GMC stepside

recently i bought the Vintage Air A/C, fitted it in

the problem im facing is that when i start the engine and get on my way, the first 15-20m its like the best cooling system, then the pressure starts getting lower and lower until air barely comes out , this is the first problem

Second problem is that after all wt i mentioned above , if the gear is on (D) and im not moving their is 0 air coming out , as soon as i move the A/C starts working again, what can be the reason behind that ?

im not really good with ac systems so need your help with that

and btw as i notice that most cars compressors goes of for a while and goes bk on so that it does not freeze inside the A/C system , My Vintage system does not turn of by time !

i had this for months now and cant wait to fix it ...