1982 yamaha seca xj750 starting problem??

i have just bought a used 1982 yamaha seca xj750. i am having trouble with it starting and stalling. %50 of the time it starts first time out but after riding for a time and i'll turn off, i'll return 10 mins later it wont start. I wait 30 mins and then it will start again. and as well the idel is ether fast or to slow. i set idel just right but after riding for just 5 mins or less the idel jumps way high. if i set the idel low and try and use the throttle to keep it up when i come to a stop it will sometimes stall out. in turn then it may not start. i have heard that the intake manifolds may be leeking some and to replace them.any other ideas for me THANK YOU!!.
why would the 30 min wait time have anything to do with it starting again?? but it does. i have replaced the battaery thinking at first it seemed that this my be it. it's not. also i can go out and start it 10,20 times today and it would start first time. it's just when i take it for a 10 or min ride or more that these things begain.
oh second ?? maybe, I have the two - 2 into 1 pipes and feel that maybe i should replace the pipes being old and some rust and all. were might i find these and is the 4 into 1 pipes better or worse.

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