i have a 68 p1800 that is great - but needs tuning in spots. it is a 4 speed with OD and has the dual SU carbs on it.

my newest issue is this - when i shift gears, especially 1st to 2nd and 2nd to 3rd, the idle shoots up. I've learned to drive the car with this issue - all i do is very lightly tap the gas pedal while the clutch is in and the idle calms down. It takes a little longer to shift from gear to gear, but lets face it: i'm in no hurry driving an 1800...
when at rest the car idles fine, very seldomly acting up. I've noticed that if i drive it hard and get the rpm's up to about 5.5 - 6 thousand before shifting the idle speed will drop normally.
now, i bought this car with a weber conversion on it, but that was bunk so i had my mechanic fit it with the dual SU's. He has also done other work - adjusted the valves, replaced the overdrive seal that was leaking trans fluid, replaced the points...

any ideas to what this quirky issue could be??? i plan on working on the car myself now ( if the task does not seem too daunting), so any advice would be great!!