92 Dodge Dakota Backfiring problem

My 92 Dakota is backfirng through the exhaust and it has gotten so bad that it blew up the muffler. You can start the truck and it will run fine sometimes but other times it will start losing power and backfiring. I replaced the plugs, distributor cap, rotary bug, and the heads have been replaced about 1 yr ago. Sometimes you can turn the key off and then back on while it is having the problems and it will clear up for about 2-3 minutes.

Yeah i forgot to mention that I change the timing chain 1yr ago when i put the new heads on it cuz the old timing chain slipped. Also it ran fine for a good 4 months and the truck set for about 7 months. Then this problem started.:banghead:

If anybody has any advice I would appreciate it if you would reply to this.

P.S. 3.9L V6 fuel injected