Temp Problems

I have an MGF 1.8 which recently has had the head gasket replaced. The car runs 100% but the oil pressure goes passed the 120 mark and then if
accelerate it cuts out. If you then stop and start the engine it is fine again. Temp gauge does not go up. No oil/water mixture. Oil perfectly clean. My thoughts are it may be the Catalyst converter but I really dont know. Please can someone shed a possible explanation. Thanks M
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