Modern Vespa

Hi All
I have had this bike from new and is my baby
I am having issues when 1st starting for the first mile it keeps loosing power , once it has run for ten mins it then seems to run ok, I have had three mechanics look at it and no luck. one fitted a second hand carb and new fuel filter , one took carb off and cleaned it sill no luck, one mechanic said due to it's age you have to run it for a while before you set off. find this difficult to accept , I have tried to find someone that will be able to fix this problem so far no luck, I am now considering getting a new Vespa , ( would really like to keep this one ) so if anyone has any ideas please help

Thanks
ps it has also have a new cold start thingy fitted

Check the valve clearances. Check your coolant for any oil in it. The head gasket might have a slight leak. It's a 21 year old bike.....

^^^^ This. You have the classic symptoms of the valve clearances needing adjustment - the 125 Leader engine is particularly susceptible to this.

Thanks for your responses , looks like valve clearance may be the problem