Cant Keep Running

I have an 89 Yamaha FJ1200.. last fall I spent 700 bucks trying to get a knocking noise fixed,, turned out to be an exhaust pipe,, they took the whole engine apart and for nothing and I had to pay them to put it all back together and get it back with the same problem,, I don't want to take it back there again,, but this year,, I can't keep the bike running,,,starts when cold,,BUT does not go into high idle when warmed with choke out,, drive a bit,, acts like it gets no gas and dies, and will not start again... changed out spark plugs,, started right up,, cold again,, ran for a short time,, till hot, died... took gas tank off, blew out filter,, put in new gas,, started right up,, cold,, when hot today,, died again didn't start up again... am letting get cold again to see if it will start again without doing anything more,, what can some one tell me is wrong,, I would like to use it this year,, did not at all last year for fear of damaging engine