A lot of thermal paste can definitely be a reaosn for high temperatures -- *all* it needs to do is fill the microscopic valleys in the metal on top of the chip & heatsink so you get rid of the air pockets -- direct metal-on-metal contact still conducts a lot better than the thermal paste itself.
It's a very common mistake to overdo it, with high temperatures to show for.
That said, my previous PC had an Athlon 2800+, and it was pretty solid at 68C, so 70 is not necesarily a problem.
My first guess would also be PSU -- over the years I've had two computers (one amiga and one PC) that had semi-random reboots, and in both cases it was a faulty PSU that caused it.
It's a very common mistake to overdo it, with high temperatures to show for.
That said, my previous PC had an Athlon 2800+, and it was pretty solid at 68C, so 70 is not necesarily a problem.
My first guess would also be PSU -- over the years I've had two computers (one amiga and one PC) that had semi-random reboots, and in both cases it was a faulty PSU that caused it.