I've got a Cleveland 4X rivet gun. Lately, the gun will not perform when facing downward. It will just "buzz" or "hiss". When placed horizontally or facing upward, it works just fine. I disassembled it and did not find any obvious problems. I did a little cleaning and lubing with tool oil and reassembled. Same problems. Supply air pressure is 100-120 psi. I purchased this gun used from The Yard 4 years ago. It had been working fine up until a few months ago. Any Thoughts ???