Hoberto,
Congrats, as others have said - great problem to have!
I think you're right to be wary, and to think things through carefully before you jump (again). Since you just came out of a lousy job in March, you know what a horrid situation that is, and how hard it can be to get out of.
You said that you really like your current co-workers and current work, and that you would be leaving them in the lurch. Why not talk to them first? Let them know that something else has just fallen into your lap, and that they're offering you about $140/hour. Ask whether they might be able to match the rate and the benefits, and ALSO ask how firm the August move looks like it will be. Obviously, being paid hourly and having limited hours available, really hurts, so if it looks like it could be indefinite, it may make sense to jump ship. But this at least lets them know there is a problem, and competition for your efforts, and it gives them a chance to come up with some creative solutions so that you can stay put with people and work you KNOW you like.
If the limited hours only last until August, and they'll give you a bit of an hourly boost to match.....heck. Enjoy your summer (or pick up some freelance on the side - sounds like you've got valuable skills).
If you decide to negotiate with the new guy, first see what you can find out about what it's like to work there. Can you ask him to talk to some of his folks? Can you ask around to see how it is? Maybe he offers a premium because it's a great place to work. Or maybe he has trouble keeping people, which isn't a good sign.
While you are at it, do a little homework on what's typical in your field, so you know what you're actually worth. Ask around, call up your university career center or academic dept., do some googling. Glass Door and other industry specific websites should be able to give you an idea of ranges of salary or hourly. As far as I know, most professions operate on EITHER salary OR hourly - the idea being that a salaried employee is often paid a bit more, but is expected to work as long as it takes to get the job done, and often to be responsive on weekends or holidays these days. An hourly employee may get a bit less per hour, but gets upside if they have to work weekends or holidays. I don't know what's typical, but you'll want to know before you negotiate so you know what to ask for, and what the criteria are for what's fair.....
Good luck!