Derailer/pedal misses "help"
 

Hi people first time poster and am back into cycling after gas went past $3.50 ga

I have a diamond back mountain bike from mid 90s in excelent shape,,,however when I ride it, about every third rotation of pedal sprocket it misses and pedals just dont engage just free pedal,,,Im sure its the derailer thats either missing or maybe gettig bumped by chain,,theres no teeth missing from any gears and its not comming from the pedals or pedal sprocket,,,does anybody have a checklist I can go through to try to pinpoint the problem
any help would be great

Sounds like your freewheel/cassette (rear gears) are gummed up internally. Need to be flushed out. Might try spraying the freewheel with penetrating lubricant.

+1 for the sticking freewheel/cassette. Try spraying the centre, near the wheel spindle with WD40, spin the cogs, do it again & so on, then try riding it again. Sometimes old lubricant inside the mechanism goes gummy. WD40 or similar eventually dissolves the sticking bits, but it often takes a few tries before it hits the right bit! Easier to do this with the wheel removed from the bike. Good luck!