Crazy Erections

[Sugarcane]

I use Edex 10 Mcg that gives a firm erection 20 min after I inject but about 15/20 min into intercourse my erection goes to 50/60%. I stop 10/15 min and I can get a 100% erection again.this cycle goes on multiple times.this kinda kills the mood for my wife. Has anyone had this problem and if so how did you correct it ?