1996 Vauxhall Corsa 1996 Corsa random bad driving

My vauxhall corsa is randomly driving very badly as it there is a blockage.I have checked spark plugs and cleaned, checked air filter and for exhaust and all pipe blockages. At first it seemed only to do this when it rained but now it seems to just randomly do it.The car does overheat now and then but this is due to a malfunction in the radiator

what is the mostly likely cause bear in mind the engine has 120k on it and is a bit oily

creamy substance on oil cap also

Also for the first time today the car wudnt start at first engine warning light came on then the car started light went out but car was drivin as if it wasnt on its correct timing then the car started to drive normal again. This is how random it is acting
somedays it is fine others it feels like its going to die

thx any help or suggestions appreciated

Thx Dave was thinking that and wondering would it be worth my while trying to repair this myself prior to refitting a used engine. The Engine would cost me about 70-100 quid and I have replaced a few before but never a head Gasket mainly brakes suspension and bits and pieces of bodywork as required.