The code P0300 is showing on the ram truck, what is causing this? on 2000 Dodge Ram 1500

I had it in the shop 8 months ago, they replaced the head gasket, iginition coils, plugs. The code went away $1700.00 later, but it is now back. I run a good oil and have it changed faithly.

Asked by for the 2000 Dodge Ram 1500
should be under warranty,so return and have them ck it out.