Need help with Engine troubles!

[csb#356]

I have a 383 stroker in my 77 pickup and I let it idle for about an hour and a half (don't ask why it's a long story). The next day it was running terrible I could hardly get it to move. I had to bring up to about 1500 rpm before letting the clutch out or it would just studder and die. It would run fine at about 2200 cruising speed. good power and very responsive just like usual but when I get to the top end at about 5000 rpm it starts backfiring. I don't see why it would all of a sudden give me timing issues so would it be a carburetor issue or problems with the valves. It has an edelbrock carburetor and performer rpm intake. Please give me your opinions.