what would cause this...?

[1BAD9D4]

ill start from the beginning. car starts up just fine with a cold start. i let the car run for a good minute or two and put car in reverse, and then my battery guage drops reading very low. ill then put it in drive and whenever i give gas the battery guage will go right up. however if i get on it hard the battery guage will go down again. ive tried replacing battery, terminals, and the alternator. any ideas? or is this normal?

 

[ivan12]

Could be a bad ground

 

[CC'S95GT]

Could be a bad ground

or corrosion inside of the battery cables

 

[94MuStAnGGT5o]

i used to have the same problem, car even turned off sometimes. I got a new battery and cleaned maf sensor and air filter, after that it ran perfectly