Given what James said in his previous post about the stat being an unknown one .... I'm inclined to agree with his conjecture that the stat may be stuck shut, and once the pressure starts to rise within the system (cap on) you are getting a localised "boiling" that is causing rapid expansion and temperature rise.
If you can leave it until James can look at it, all well and good.....
If you want to do more diagnostics .. try this .... and if you do it with the cap off it will be safer .....
Start engine from cold .. preferably overnight cold ... allow to idle and stand with one hand on the top hose where it comes from the thermostat. That pipe should remain cold until the temp gauge gets to mid 80's at which point you should feel a sudden "surge" of hot water as the thermostat opens.
If the water warms up slowly throughout, the stat is stuck shut
If the water does not "surge" but seems to heat up from the radiator end "backwards" so to speak, the stat is stuck shut and you are feeling the thermal siphon effect
HTH