I agree with the others on checking your mixing valve. If it's not already turn all the way up you might want to try turning it up a bit. If it helps that might be the culprit.
Another option on a tank would be something like this tank from
http://www.vaughncorp.com/SEPCO.pdf
I installed one a few years ago. It has two heat exchangers and electric. I reserved one heat exchanger for a future solar installation, the other I use off my pellet stove. You could use one for solar and the second one off your existing boiler. You could then switch to the electric as a primary or back-up source. It would give you a lot of choices for heating your domestic hot water depending on the season or fuel cost.