Sounds like exactly the same problem I had when I got an Ion for the first time this week. Fixed by stripping the whole marker down, lubing all the bits that need lube, cleaning those that don't and rebuilding. The bolt & reg in particular needed some lovin.
Disassembly video here:
http://youtube.com/watch?v=cB1LtKY9jQE
Disassembly steps and lube guide for bolt here:
http://www.zdspb.com/tech/mguide/ion/maintenance_bolt_stock.html
Solenoid strip & clean here:
http://www.zdspb.com/tech/mguide/ion/maintenance_solenoid.html
Reg strip and clean here:
http://www.zdspb.com/tech/mguide/regs/main_ionreg.html
Do all those, reassemble, turn the reg all the way down before regassing, then slowly crank the velocity up while firing and it should run fine. Don't exceed 200psi input pressure.
Make sure you use a fresh 9v, the Ion solenoid is a bit agricultural and needs the oomph of a fresh, decent make battery (Duracell Plus/Procell/Duracell Ultra recommended).
Only 'specialist' tool required is a 9/16" deep well socket for the reg, however I found a bog standard 14mm socket worked if I didn't click the ratchet all the way in.
When re-installing the board, make sure you route the battery wires BEHIND the board (towards the rear of the grip frame) and don't get them jammed (like I did, mangling one to buggery), otherwise they tend to interfere with the trigger switch.
Easy enough to do, I went from never having touched an Ion before to a complete rebuild in an hour, they aren't complicated, just weird.
