Having A Problem With Running Lights?

I have A 1989 Pontiac Firebird T- top. It is a basic model with a 305 TBI.(vin code E) Automatic. It all started about a month ago when i was going down my dirt road and it was at night and i hit a little bump and I heard a fuse pop. The next morning went out to find out the problem and noticed that it was my running lights. I replaced the fuse and when i did it popped right away. It done this about three times and finnally i went and took the tail lights off and checked the bulbs and two were bad, so i replaced them. and the ground wasnt good so I fixed that as well and then i tried the fuse again and it worked perfect.
It has worked perfect for about a month now, until this past sunday and i was on the dirt road again, hit another bump and the same fuse poped. It was dark so I was using my lights.

I have checked my ground in the back and the two right behind each headlight. Everything was fine , except for there yet again two bulbs blew in the back so i replaced them and it still is popping fuses. It calls for a 20amp fuse, I have even tryied a 25amp fuse, and it pops those as well. The turn signals still work and the stop lights still work but not the running lights in the front or rear. I have racked my brain and I am clueless at this point!