Thanks everyone for the replies, I have now found a solution so I'll
answer all questions here.

> Do you have a resistor to MCLR?

1. Yes, I had a 10k-resistor to MCLR. I wrote about it in the OP but I
see now it was very vague described.

> Do you have decoupling caps?

2. No, I had none (shame on me!) but I didn't on the breadboard
either. Temporarily added one (it's a real pain to add decouplers when
using veroboard and Vcc/GND-pins are on opposite sides on the IC).

> Why do you use MCLR? Disable it and save a pin.

3. I have always used MCLR since I have read a lot of problems that
might arise when not using it: programming could lock me out from the
chip, etc, so I've always used it to be on the safe side. Since the
project is mature I don't need an extra pin either.

> Touch the pin with a dielectric to check cold soldering.

4. Did this. Nothing happened when I touched with plastic.


So I investigated a bit further to see why I had missed. I tried to
unhook the Wisp648 from the breadboard, and this way the breadboard
version behaves the same as the PCB-version. Putting back the
MCLR-cable from the Wisp and everything worked fine.

After this I disabled MCLR to see if that helped. It did not (with or
without decouplers). Therefore I read more about the power up-timer in
the datasheet and enabled it after suggested here.

Voila! That timer really did its job - everything works now (with or
without MCLR). Thanks for suggesting it.

However, I'm still pretty curious to find out why the original problem
appeared. I have always used MCLR for my projects, never had any
problems. Never been using 16F628a before, though. Maybe that chip
requires special attention, I do not know. But what I do know is that
I've always used the same PSU for my breadboard projects, so the "slow
powerup" should have been visible earlier.

Unfortunately I haven't got a scope, and cannot afford one, so I can't
have a look of what's really happening with the Vdd/MCLR-pins. In the
future, all my 628a:s will have PWRT enabled.


-- 
- Rikard - http://bos.hack.org/cv/
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