What was found
INTERMITTENT "GPS- INVALID" SCRATCH PAD MESSAGE APPEARED ALONE AND SEEMINGLY ASSOCIATED WITH INVALID MSG. CPDLC DURING THIS TIME WAS VERY SLOW TO RESPOND/ REPLY TO ATC QUERIES/ DIRECTIVES. (NEVER DID THE VTK FLAG APPEAR.). GROUNDED - YES. DIVERSION TIME 78 MINUTES - C/W B737-800 AMM 34-58-730-802 GPS SYSTEM TEST NO DEFECTS NOTED, C/W HF 1 AND 2 COMM SYS TEST REF AMM 23-11-00-730-801 SYSTEM CK GOOD. C/W AMM 34-53-00-710-801 XPNDR OPS TEST GOOD. C/W CPDLC FMC FUNCTIONAL TEST SYSTEM TEST GOOD.