OK, It is rare that I get to help out, but this is my take on it...
1) check your little clamps on the intalke boots, Black or red. Check to see if they are stripped (clamps). The cheep ones they send in the boot kit are CRAP. The ones on the ASBug stripped out and I would get intermittent vaccume leaks as every thing heated up. If they are stripped, take one to a FLAPS and get a radiator clamp and use it.
If the clamps are tight and not stripped out, then check the idle circut on the base of the carb on the right side on the bottom, it'll look lie it has a BB in it and is brass. Loosen it slightly to see if the idle gets better. If the idle improves then take it out and look at it. Is it nasty? does it have garbage on it? if so it is probbably plugged and needs the carb cleaner throught the WD40 red straw blown throught it.
If this isn't doing the trick, (My ASBug ran great with a new 34 on it for like 6 months and then started doing what your describing...) look at a new carb then...
2) What distributor are you using? if it is an 009, 010 or other non vaccume actuated one (w/o the vaccume can), the 34 has a reputation of not liking those. That is what is in my Jap Beetle now and it is a noticbly hessitant...
2) The 30/31 CAN be used on your 1600 DP (If you order the one with the adaptor.) It is a smaller bore and will give a little less speed top end, but should cause your MPG to go up. Just make sure to order the one with the adaptor.
Let us know what you find....
KC