So, I recently bought my first bike. An 06 Suzuki Boulevard S40.
I bought it used with 1,700 miles. Ever since I had it I've had problems.
The ongoing problem though is that my bike will just randomly shut off while I ride.
I took it to a mechanic when the whole battery would shut off. They said I had loose connectors and tightened them. On my way home from the shop it shut off, so the store came and trailered it and did more work on it. They told me it was the fuel supply valve. There was some part in it that the mechanic hadn't seen before and said wasn't suppose to be there, so he took it out and told me it was fine. It worked for about two days and then shut off while I was riding it again.
What I didn't notice is that the mechanic had put my petcock on RES. So I switched it back on the day it shut off before I took it out.
Later when I would try and start my bike it wouldn't even keep an idle, even after having my choke on for about 3-5 minutes.  

I turned my petcock back to RES and it seemed to run more steadily, just idled lower. But I took my bike out today and made it about two miles before it shut off. I pulled over to the side of the road and after a minute or two and a couple tries to start it, it was able to keep an idle.

It started after I took my gas cap off though. I'm guessing that there just isn't any ventilation getting in causing it to have a vacuum effect?
That paired with maybe the fuel line for ON is clogged a bit?
I did have someone go through and flush all the lines out and did regular maintenance  work to it since the bike had been sitting for so long.

Otherwise the only common thing that happens when it shuts off is that it happens when my clutch is in. Usually just while I change gears, I'll go to give it gas and it will just click off.

Any suggestions?
I really don't want to have to sell it, but I can't seem to get it fixed and don't have money to keep taking it back to the shop :'(

HELP ME PLEASEEE!!

unfortunately I didn't keep it =[. I figured the mechanic knew what he was doing and didn't even think about asking for it.
thank you for the suggestions, i will definitely do that!

#1 rule around here... never assume the mechanic knows what he's doing.  You don't have to tell them that, but watch, listen, ask questions, investigate.

when it dies on ya, put it to prime.  that bypasses the vacuum on/off switch, just remember to put to "on" when you shut it off.

good chance that it's the vac line, but do bot's petcock check for sure.

lesson learned.
I'm going to try and get my bike over to my friends ASAP where all the necessary equipment is. hopefully within the next few days. I'll definitely let all of you know how it went.
thank you!