I had exactly the same problems with my son from the time he was about 3 months old. He would wake up after 30 minutes on the dot - I could set me clock by him
The only thing that worked for us was for me to creep into his room at the 25 minute mark and sit beside his cot, with my hand lightly on his chest & arms (he has always been wrapped for his naps). It was the jolt that was waking him up and he couldn't get back to sleep after this - by resting my hand on him, I seemed to lessen the jolt and he didn't stir as much, and could go into the next sleep cycle. It took a while, sometimes I was sitting there for 15 minutes before he was soundly asleep into the next cycle. We did this for a few weeks, it took a lot of time & effort but it worked for us. By the time he was 4 1/2 months old, he could nap for longer on his own.
HTH