Fuel pump going crazy
 

2000 K3500 7.4l
Just noticed it.
Turn key on, gauge shows full.
Start engine, still full.
Run for around 5 minutes, or more, and it drops to empty.
A few seconds later it goes back to full.
It's a random thing.
The fuel pump was changed about a year ago.
I think it was an Airtex aftermarket pump.
Any ideas????