are u using OEM fuel pump? Aftermarket pumps are junk. use OEM if you want them to last, that or keep replacing aftermarket (BTDT!)
Check your grounds too, and do a loaded circuit voltage drop test on all of the wirings. These things are well known for wiring problems. One main reason I put a real alternator on mine and ditched the dynamo. I got one from a BX1860 I think it was. Had to do some wiring but wasn't too bad. I just used the original wiring from the regulator and modified as needed (ign wire specifically). Run a new hot wire from the charge post to the starter, the original won't handle the 40 some amps of current and will burn up. Other'n that it mostly bolts on. It actually charges too. Well so does the dynamo but the dynamo doesn't put out a lot of amps in comparison. Especially at idle and low speeds.