How to fix the problem with turning on the computer?

Question

motherboards

Hello!
I have an old computer that sometimes just stops working. The problem is expressed as follows: you turn it on, the fans start spinning, but there is no picture on the monitor. I remove all the RAM, insert it again and the computer continues to work on until one fine day it starts all over again. I'm already used to it, if not for one thing, I recently bought a new computer and one day the same thing happened.
The memory was checked with memtest, no errors were found. Maybe someone has encountered such a problem and knows why it occurs and how to solve it once and for all, and not just rearrange the memory?
The motherboard of an old computer Asus P5K-E
The motherboard of a new computer Asus Z87-K
Memory Kingston (in the old computer 2 slots of 1GB, in the new 2 slots of 8GB)

Answer 1

The reason is a bad connection somewhere. Not necessarily in memory slots. They took out and inserted the memory - the motherboard moved, the contact appeared. It is worth taking out and inserting all the connectors.
There is such a thing - let's say a person buys another car, and the same problem appears in it as in the old one.